new terms Flashcards

Question

atomic

Answer 1

In the SQL context, transactions are units of work that either succeed entirely (when committed) or have no effect at all (when rolled back). The indivisible ("atomic") property of transactions is the "A" in the acronym ACID.

Answer 2

Special instructions provided by the CPU, to ensure that critical low-level operations cannot be interrupted.

Answer 3

A property of a table column (specified by the AUTO\_INCREMENT keyword) that automatically adds an ascending sequence of values in the column. InnoDB supports auto-increment only for primary key columns. It saves work for the developer, not to have to produce new unique values when inserting new rows. It provides useful information for the query optimizer, because the column is known to be not null and with unique values. The values from such a column can be used as lookup keys in various contexts, and because they are auto-generated there is no reason to ever change them; for this reason, primary key columns are often specified as auto-incrementing. Auto-increment columns can be problematic with statement-based replication, because replaying the statements on a slave might not produce the same set of column values as on the master, due to timing issues. When you have an auto-incrementing primary key, you can use statement-based replication only with the setting innodb\_autoinc\_lock\_mode=1. If you have innodb\_autoinc\_lock\_mode=2, which allows higher concurrency for insert operations, use row-based replication rather than statement-based replication. The setting innodb\_autoinc\_lock\_mode=0 is the previous (traditional) default setting and should not be used except for compatibility purposes.

Answer 4

The convenience of an auto-increment primary key involves some tradeoff with concurrency. In the simplest case, if one transaction is inserting values into the table, any other transactions must wait to do their own inserts into that table, so that rows inserted by the first transaction receive consecutive primary key values. InnoDB includes optimizations, and the innodb\_autoinc\_lock\_mode option, so that you can choose how to trade off between predictable sequences of auto-increment values and maximum concurrency for insert operations.

Answer 5

A setting that causes a commit operation after each SQL statement. This mode is not recommended for working with InnoDB tables withtransactions that span several statements. It can help performance for read-only transactions on InnoDB tables, where it minimizes overhead from locking and generation of undo data, especially in MySQL 5.6.4 and up. It is also appropriate for working with MyISAM tables, where transactions are not applicable.

Answer 6

The ability to cope with, and if necessary recover from, failures on the host, including failures of MySQL, the operating system, or the hardware and maintenance activity that may otherwise cause downtime. Often paired with scalability as critical aspects of a large-scale deployment.

Answer 7

A tree data structure that is popular for use in database indexes. The structure is kept sorted at all times, enabling fast lookup for exact matches (equals operator) and ranges (for example, greater than, less than, and BETWEEN operators). This type of index is available for most storage engines, such as InnoDB and MyISAM. Because B-tree nodes can have many children, a B-tree is not the same as a binary tree, which is limited to 2 children per node. Contrast with hash index, which is only available in the MEMORY storage engine. The MEMORY storage engine can also use B-tree indexes, and you should choose B-tree indexes for MEMORY tables if some queries use range operators.

Answer 8

Identifiers within MySQL SQL statements must be quoted using the backtick character (`) if they contain special characters or reserved words. For example, to refer to a table named FOO#BAR or a column named SELECT, you would specify the identifiers as `FOO#BAR` and `SELECT`. Since the backticks provide an extra level of safety, they are used extensively in program-generated SQL statements, where the identifier names might not be known in advance. Many other database systems use double quotation marks (") around such special names. For portability, you can enable ANSI\_QUOTES mode in MySQL and use double quotation marks instead of backticks to qualify identifier names.

Answer 9

The process of copying some or all table data and metadata from a MySQL instance, for safekeeping. Can also refer to the set of copied files. This is a crucial task for DBAs. The reverse of this process is the restore operation. With MySQL, physical backups are performed by the MySQL Enterprise Backup product, and logical backups are performed by the mysqldump command. These techniques have different characteristics in terms of size and representation of the backup data, and speed (especially speed of the restore operation). Backups are further classified as hot, warm, or cold depending on how much they interfere with normal database operation. (Hot backups have the least interference, cold backups the most.)

Answer 10

The code name for an InnoDB file format that supports compression for table data. This file format was first introduced in the InnoDB Plugin. It supports the compressed row format that enables InnoDB table compression, and the dynamic row format that improves the storage layout for BLOB and large text columns. You can select it through the innodb\_file\_format option. Because the InnoDB system tablespace is stored in the original Antelope file format, to use the Barracuda file format you must also enable the file-per-table setting, which puts newly created tables in their own tablespaces separate from the system tablespace. The MySQL Enterprise Backup product version 3.5 and above supports backing up tablespaces that use the Barracuda file format.

Answer 11

A non-generated table column upon which a stored generated column or virtual generated column is based. In other words, a base column is a non-generated table column that is part of a generated column definition.

Answer 12

An early stage in the life of a software product, when it is available only for evaluation, typically without a definite release number or a number less than 1. InnoDB does not use the beta designation, preferring an early adopter phase that can extend over several point releases, leading to a GA release.

Answer 13

A file containing a record of all statements that attempt to change table data. These statements can be replayed to bring slave servers up to date in a replication scenario, or to bring a database up to date after restoring table data from a backup. The binary logging feature can be turned on and off, although Oracle recommends always enabling it if you use replication or perform backups. You can examine the contents of the binary log, or replay those statements during replication or recovery, by using the mysqlbinlog command. For full information about the binary log, see Section 5.2.4, “The Binary Log”. For MySQL configuration options related to the binary log, see Section 17.1.4.4, “Binary Log Options and Variables”. For the MySQL Enterprise Backup product, the file name of the binary log and the current position within the file are important details. To record this information for the master server when taking a backup in a replication context, you can specify the --slave-info option. Prior to MySQL 5.0, a similar capability was available, known as the update log. In MySQL 5.0 and higher, the binary log replaces the update log.

Answer 14

An informal name for the binary log file. For example, you might see this abbreviation used in e-mail messages or forum discussions.

Answer 15

A special mode of full-text search enabled by the WITH QUERY EXPANSION clause. It performs the search twice, where the search phrase for the second search is the original search phrase concatenated with the few most highly relevant documents from the first search. This technique is mainly applicable for short search phrases, perhaps only a single word. It can uncover relevant matches where the precise search term does not occur in the document.

Answer 16

An SQL data type (TINYBLOB, BLOB, MEDIUMBLOB, and LONGBLOB) for objects containing any kind of binary data, of arbitrary size. Used for storing documents, images, sound files, and other kinds of information that cannot easily be decomposed to rows and columns within a MySQL table. The techniques for handling BLOBs within a MySQL application vary with each Connector and API. MySQL Connector/ODBC defines BLOB values as LONGVARBINARY. For large, free-form collections of character data, the industry term is CLOB, represented by the MySQL TEXT data types.

Answer 17

A portion of a system that is constrained in size or capacity, that has the effect of limiting overall throughput. For example, a memory area might be smaller than necessary; access to a single required resource might prevent multiple CPU cores from running simultaneously; or waiting for disk I/O to complete might prevent the CPU from running at full capacity. Removing bottlenecks tends to improve concurrency. For example, the ability to have multiple InnoDB buffer pool instances reduces contention when multiple sessions read from and write to the buffer pool simultaneously.

Answer 18

A shutdown operation immediately followed by a restart. Ideally with a relatively short warmup period so that performance and throughput quickly return to a high level.

Answer 19

A mechanism for managing different-sized pages in the InnoDB buffer pool.

Answer 20

A memory or disk area used for temporary storage. Data is buffered in memory so that it can be written to disk efficiently, with a few large I/O operations rather than many small ones. Data is buffered on disk for greater reliability, so that it can be recovered even when a crash or other failure occurs at the worst possible time. The main types of buffers used by InnoDB are the buffer pool, the doublewrite buffer, and the insert buffer.

Answer 21

The memory area that holds cached InnoDB data for both tables and indexes. For efficiency of high-volume read operations, the buffer pool is divided into pages that can potentially hold multiple rows. For efficiency of cache management, the buffer pool is implemented as a linked list of pages; data that is rarely used is aged out of the cache, using a variation of the LRU algorithm. On systems with large memory, you can improve concurrency by dividing the buffer pool into multiple buffer pool instances. Several InnoDB status variables, information\_schema tables, and performance\_schema tables help to monitor the internal workings of the buffer pool. Starting in MySQL 5.6, you can also dump and restore the contents of the buffer pool, either automatically during shutdown and restart, or manually at any time, through a set of InnoDB configuration variables such as innodb\_buffer\_pool\_dump\_at\_shutdown and innodb\_buffer\_pool\_load\_at\_startup.

Answer 22

Any of the multiple regions into which the buffer pool can be divided, controlled by the innodb\_buffer\_pool\_instancesconfiguration option. The total memory size specified by the innodb\_buffer\_pool\_size is divided among all the instances. Typically, multiple buffer pool instances are appropriate for systems devoting multiple gigabytes to the InnoDB buffer pool, with each instance 1 gigabyte or larger. On systems loading or looking up large amounts of data in the buffer pool from many concurrent sessions, having multiple instances reduces the contention for exclusive access to the data structures that manage the buffer pool.

Answer 23

The built-in InnoDB storage engine within MySQL is the original form of distribution for the storage engine. Contrast with the InnoDB Plugin. Starting with MySQL 5.5, the InnoDB Plugin is merged back into the MySQL code base as the built-in InnoDB storage engine (known as InnoDB 1.1). This distinction is important mainly in MySQL 5.1, where a feature or bug fix might apply to the InnoDB Plugin but not the built-in InnoDB, or vice versa.

Answer 24

The relationships and sequences of actions that form the basis of business software, used to run a commercial company. Sometimes these rules are dictated by law, other times by company policy. Careful planning ensures that the relationships encoded and enforced by the database, and the actions performed through application logic, accurately reflect the real policies of the company and can handle real-life situations. For example, an employee leaving a company might trigger a sequence of actions from the human resources department. The human resources database might also need the flexibility to represent data about a person who has been hired, but not yet started work. Closing an account at an online service might result in data being removed from a database, or the data might be moved or flagged so that it could be recovered if the account is re-opened. A company might establish policies regarding salary maximums, minimums, and adjustments, in addition to basic sanity checks such as the salary not being a negative number. A retail database might not allow a purchase with the same serial number to be returned more than once, or might not allow credit card purchases above a certain value, while a database used to detect fraud might allow these kinds of things.

Answer 25

The general term for any memory area that stores copies of data for frequent or high-speed retrieval. In InnoDB, the primary kind of cache structure is the buffer pool.

Answer 26

The number of different values in a table column. When queries refer to columns that have an associated index, the cardinality of each column influences which access method is most efficient. For example, for a column with a unique constraint, the number of different values is equal to the number of rows in the table. If a table has a million rows but only 10 different values for a particular column, each value occurs (on average) 100,000 times. A query such as SELECT c1 FROM t1 WHERE c1 = 50; thus might return 1 row or a huge number of rows, and the database server might process the query differently depending on the cardinality of c1. If the values in a column have a very uneven distribution, the cardinality might not be a good way to determine the best query plan. For example, SELECT c1 FROM t1 WHERE c1 = x; might return 1 row when x=50 and a million rows when x=30. In such a case, you might need to use index hints to pass along advice about which lookup method is more efficient for a particular query. Cardinality can also apply to the number of distinct values present in multiple columns, as in a composite index. For InnoDB, the process of estimating cardinality for indexes is influenced by the innodb\_stats\_sample\_pages and the innodb\_stats\_on\_metadata configuration options. The estimated values are more stable when the persistent statistics feature is enabled (in MySQL 5.6 and higher).

Answer 27

A special data structure that records changes to pages in secondary indexes. These values could result from SQL INSERT, UPDATE, or DELETE statements (DML). The set of features involving the change buffer is known collectively as change buffering, consisting of insert buffering, delete buffering, and purge buffering. Changes are only recorded in the change buffer when the relevant page from the secondary index is not in the buffer pool. When the relevant index page is brought into the buffer pool while associated changes are still in the change buffer, the changes for that page are applied in the buffer pool (merged) using the data from the change buffer. Periodically, the purge operation that runs during times when the system is mostly idle, or during a slow shutdown, writes the new index pages to disk. The purge operation can write the disk blocks for a series of index values more efficiently than if each value were written to disk immediately. Physically, the change buffer is part of the system tablespace, so that the index changes remain buffered across database restarts. The changes are only applied (merged) when the pages are brought into the buffer pool due to some other read operation. The kinds and amount of data stored in the change buffer are governed by the innodb\_change\_buffering and innodb\_change\_buffer\_max\_size configuration options. To see information about the current data in the change buffer, issue the SHOW ENGINE INNODB STATUS command. Formerly known as the insert buffer.

Answer 28

The general term for the features involving the change buffer, consisting of insert buffering, delete buffering, and purge buffering. Index changes resulting from SQL statements, which could normally involve random I/O operations, are held back and performed periodically by a background thread. This sequence of operations can write the disk blocks for a series of index values more efficiently than if each value were written to disk immediately. Controlled by the innodb\_change\_buffering and innodb\_change\_buffer\_max\_size configuration options.

Answer 29

As changes are made to data pages that are cached in the buffer pool, those changes are written to the data files sometime later, a process known as flushing. The checkpoint is a record of the latest changes (represented by an LSN value) that have been successfully written to the data files.

Answer 30

In InnoDB, a validation mechanism to detect corruption when a page in a tablespace is read from disk into the InnoDB buffer pool. This feature is turned on and off by the innodb\_checksums configuration option. In MySQL 5.6, you can enable a faster checksum algorithm by also specifying the configuration option innodb\_checksum\_algorithm=crc32. The innochecksum command helps to diagnose corruption problems by testing the checksum values for a specified tablespace file while the MySQL server is shut down. MySQL also uses checksums for replication purposes. For details, see the configuration options binlog\_checksum, master\_verify\_checksum, and slave\_sql\_verify\_checksum.

Answer 31

In a foreign key relationship, a child table is one whose rows refer (or point) to rows in another table with an identical value for a specific column. This is the table that contains the FOREIGN KEY ... REFERENCES clause and optionally ON UPDATE and ON DELETE clauses. The corresponding row in the parent table must exist before the row can be created in the child table. The values in the child table can prevent delete or update operations on the parent table, or can cause automatic deletion or updates in the child table, based on the ON CASCADEoption used when creating the foreign key.

Answer 32

A page in the InnoDB buffer pool where all changes made in memory have also been written (flushed) to the data files. The opposite of a dirty page.

Answer 33

A shutdown that completes without errors and applies all changes to InnoDB tables before finishing, as opposed to a crash or a fast shutdown. Synonym for slow shutdown.

Answer 34

A type of program that sends requests to a server, and interprets or processes the results. The client software might run only some of the time (such as a mail or chat program), and might run interactively (such as the mysql command processor).

Answer 35

Files containing collections of functions for working with databases. By compiling your program with these libraries, or installing them on the same system as your application, you can run a database application (known as a client) on a machine that does not have the MySQL server installed; the application accesses the database over a network. With MySQL, you can use the libmysqlclient library from the MySQL server itself.

Answer 36

A type of prepared statement where the caching and reuse are managed locally, emulating the functionality of server-side prepared statements. Historically, used by some Connector/J, Connector/ODBC, and Connector/PHP developers to work around issues with server-side stored procedures. With modern MySQL server versions, server-side prepared statements are recommended for performance, scalability, and memory efficiency.

Answer 37

An SQL data type (TINYTEXT, TEXT, MEDIUMTEXT, or LONGTEXT) for objects containing any kind of character data, of arbitrary size. Used for storing text-based documents, with associated character set and collation order. The techniques for handling CLOBs within a MySQL application vary with each Connector and API. MySQL Connector/ODBC defines TEXT values as LONGVARCHAR. For storing binary data, the equivalent is the BLOB type.

Answer 38

The InnoDB term for a primary key index. InnoDB table storage is organized based on the values of the primary key columns, to speed up queries and sorts involving the primary key columns. For best performance, choose the primary key columns carefully based on the most performance-critical queries. Because modifying the columns of the clustered index is an expensive operation, choose primary columns that are rarely or never updated. In the Oracle Database product, this type of table is known as an index-organized table.

Answer 39

A backup taken while the database is shut down. For busy applications and web sites, this might not be practical, and you might prefer a warm backup or a hot backup.

Answer 40

A data item within a row, whose storage and semantics are defined by a data type. Each table and index is largely defined by the set of columns it contains. Each column has a cardinality value. A column can be the primary key for its table, or part of the primary key. A column can be subject to a unique constraint, a NOT NULL constraint, or both. Values in different columns, even across different tables, can be linked by a foreign key relationship. In discussions of MySQL internal operations, sometimes field is used as a synonym.

Answer 41

An index on a single column.

Answer 42

When an index is created with a length specification, such as CREATE INDEX idx ON t1 (c1(N)), only the first N characters of the column value are stored in the index. Keeping the index prefix small makes the index compact, and the memory and disk I/O savings help performance. (Although making the index prefix too small can hinder query optimization by making rows with different values appear to the query optimizer to be duplicates.) For columns containing binary values or long text strings, where sorting is not a major consideration and storing the entire value in the index would waste space, the index automatically uses the first N (typically 768) characters of the value to do lookups and sorts.

Answer 43

Synonym for statement interceptor. One aspect of the interceptor design pattern available for both Connector/NET and Connector/J. What Connector/NET calls a command, Connector/J refers to as a statement. Contrast with exception interceptor.

Answer 44

A SQL statement that ends a transaction, making permanent any changes made by the transaction. It is the opposite of rollback, which undoes any changes made in the transaction. InnoDB uses an optimistic mechanism for commits, so that changes can be written to the data files before the commit actually occurs. This technique makes the commit itself faster, with the tradeoff that more work is required in case of a rollback. By default, MySQL uses the autocommit setting, which automatically issues a commit following each SQL statement.

Answer 45

The default InnoDB row format since MySQL 5.0.3. Available for tables that use the Antelope file format. It has a more compact representation for nulls and variable-length fields than the prior default (redundant row format). Because of the B-tree indexes that make row lookups so fast in InnoDB, there is little if any performance benefit to keeping all rows the same size. For additional information about InnoDB COMPACT row format, see Section 14.9.4, “COMPACT and REDUNDANT Row Formats”.

Answer 46

An index that includes multiple columns.

Answer 47

The compression feature of the MySQL Enterprise Backup product makes a compressed copy of each tablespace, changing the extension from .ibd to .ibz. Compressing the backup data allows you to keep more backups on hand, and reduces the time to transfer backups to a different server. The data is uncompressed during the restore operation. When a compressed backup operation processes a table that is already compressed, it skips the compression step for that table, because compressing again would result in little or no space savings. A set of files produced by the MySQL Enterprise Backup product, where each tablespace is compressed. The compressed files are renamed with a .ibz file extension. Applying compression right at the start of the backup process helps to avoid storage overhead during the compression process, and to avoid network overhead when transferring the backup files to another server. The process of applying the binary log takes longer, and requires uncompressing the backup files.

Answer 48

A row format that enables data and index compression for InnoDB tables. It was introduced in the InnoDB Plugin, available as part of the Barracuda file format. Large fields are stored away from the page that holds the rest of the row data, as in dynamic row format. Both index pages and the large fields are compressed, yielding memory and disk savings. Depending on the structure of the data, the decrease in memory and disk usage might or might not outweigh the performance overhead of uncompressing the data as it is used. See Section 14.7, “InnoDB Compressed Tables” for usage details. For additional information about InnoDB COMPRESSED row format, see Section 14.9.3, “DYNAMIC and COMPRESSED Row Formats”.

Answer 49

A table for which the data is stored in compressed form. For InnoDB, it is a table created with ROW\_FORMAT=COMPRESSED. SeeSection 14.7, “InnoDB Compressed Tables” for more information.

Answer 50

A feature with wide-ranging benefits from using less disk space, performing less I/O, and using less memory for caching. InnoDB table and index data can be kept in a compressed format during database operation. The data is uncompressed when needed for queries, and re-compressed when changes are made by DML operations. After you enable compression for a table, this processing is transparent to users and application developers. DBAs can consult information\_schema tables to monitor how efficiently the compression parameters work for the MySQL instance and for particular compressed tables. When InnoDB table data is compressed, the compression applies to the table itself, any associated index data, and the pages loaded into the buffer pool. Compression does not apply to pages in the undo buffer. The table compression feature requires using MySQL 5.5 or higher, or the InnoDB Plugin in MySQL 5.1 or earlier, and creating the table using the Barracuda file format and compressed row format, with the innodb\_file\_per\_table setting turned on. The compression for each table is influenced by the KEY\_BLOCK\_SIZE clause of the CREATE TABLE and ALTER TABLE statements. In MySQL 5.6 and higher, compression is also affected by the server-wide configuration options innodb\_compression\_failure\_threshold\_pct, innodb\_compression\_level, and innodb\_compression\_pad\_pct\_max. See Section 14.7, “InnoDB Compressed Tables” for usage details. Another type of compression is the compressed backup feature of the MySQL Enterprise Backup product.

Answer 51

Not actually an error, rather an expensive operation that can occur when using compression in combination with DML operations. It occurs when: updates to a compressed page overflow the area on the page reserved for recording modifications; the page is compressed again, with all changes applied to the table data; the re-compressed data does not fit on the original page, requiring MySQL to split the data into two new pages and compress each one separately. To check the frequency of this condition, query the table INFORMATION\_SCHEMA.INNODB\_CMPand check how much the value of the COMPRESS\_OPS column exceeds the value of the COMPRESS\_OPS\_OK column. Ideally, compression failures do not occur often; when they do, you can adjust the configuration options innodb\_compression\_level,innodb\_compression\_failure\_threshold\_pct, and innodb\_compression\_pad\_pct\_max.

Answer 52

See composite index.

Answer 53

The ability of multiple operations (in database terminology, transactions) to run simultaneously, without interfering with each other. Concurrency is also involved with performance, because ideally the protection for multiple simultaneous transactions works with a minimum of performance overhead, using efficient mechanisms for locking.

Answer 54

The file that holds the option values used by MySQL at startup. Traditionally, on Linux and UNIX this file is named my.cnf, and on Windows it is named my.ini. You can set a number of options related to InnoDB under the [mysqld] section of the file. Typically, this file is searched for in the locations /etc/my.cnf /etc/mysql/my.cnf /usr/local/mysql/etc/my.cnf and ~/.my.cnf. See Section 4.2.6, “Using Option Files” for details about the search path for this file. When you use the MySQL Enterprise Backup product, you typically use two configuration files: one that specifies where the data comes from and how it is structured (which could be the original configuration file for your real server), and a stripped-down one containing only a small set of options that specify where the backup data goes and how it is structured. The configuration files used with the MySQL Enterprise Backup product must contain certain options that are typically left out of regular configuration files, so you might need to add some options to your existing configuration file for use with MySQL Enterprise Backup.

Answer 55

A read operation that uses snapshot information to present query results based on a point in time, regardless of changes performed by other transactions running at the same time. If queried data has been changed by another transaction, the original data is reconstructed based on the contents of the undo log. This technique avoids some of the locking issues that can reduce concurrency by forcing transactions to wait for other transactions to finish. With the repeatable read isolation level, the snapshot is based on the time when the first read operation is performed. With the read committed isolation level, the snapshot is reset to the time of each consistent read operation. Consistent read is the default mode in which InnoDB processes SELECT statements in READ COMMITTED and REPEATABLE READ isolation levels. Because a consistent read does not set any locks on the tables it accesses, other sessions are free to modify those tables while a consistent read is being performed on the table. For technical details about the applicable isolation levels, see Section 14.2.4, “Consistent Nonlocking Reads”.

Answer 56

An automatic test that can block database changes to prevent data from becoming inconsistent. (In computer science terms, a kind of assertion related to an invariant condition.) Constraints are a crucial component of the ACID philosophy, to maintain data consistency. Constraints supported by MySQL include FOREIGN KEY constraints and unique constraints.

Answer 57

A value that is incremented by a particular kind of InnoDB operation. Useful for measuring how busy a server is, troubleshooting the sources of performance issues, and testing whether changes (for example, to configuration settings or indexes used by queries) have the desired low-level effects. Different kinds of counters are available through performance\_schema tables and information\_schema tables, particularlyinformation\_schema.innodb\_metrics.

Answer 58

An index that includes all the columns retrieved by a query. Instead of using the index values as pointers to find the full table rows, the query returns values from the index structure, saving disk I/O. InnoDB can apply this optimization technique to more indexes than MyISAM can, because InnoDB secondary indexes also include the primary key columns. InnoDB cannot apply this technique for queries against tables modified by a transaction, until that transaction ends. Any column index or composite index could act as a covering index, given the right query. Design your indexes and queries to take advantage of this optimization technique wherever possible.

Answer 59

A type of workload where the primary bottleneck is CPU operations in memory. Typically involves read-intensive operations where the results can all be cached in the buffer pool.

Answer 60

MySQL uses the term "crash" to refer generally to any unexpected shutdown operation where the server cannot do its normal cleanup. For example, a crash could happen due to a hardware fault on the database server machine or storage device; a power failure; a potential data mismatch that causes the MySQL server to halt; a fast shutdown initiated by the DBA; or many other reasons. The robust, automatic crash recovery for InnoDB tables ensures that data is made consistent when the server is restarted, without any extra work for the DBA.

Answer 61

The cleanup activities that occur when MySQL is started again after a crash. For InnoDB tables, changes from incomplete transactions are replayed using data from the redo log. Changes that were committed before the crash, but not yet written into the data files, are reconstructed from the doublewrite buffer. When the database is shut down normally, this type of activity is performed during shutdown by the purge operation. During normal operation, committed data can be stored in the change buffer for a period of time before being written to the data files. There is always a tradeoff between keeping the data files up-to-date, which introduces performance overhead during normal operation, and buffering the data, which can make shutdown and crash recovery take longer.

Answer 62

Acronym for "create, read, update, delete", a common sequence of operations in database applications. Often denotes a class of applications with relatively simple database usage (basic DDL, DML and query statements in SQL) that can be implemented quickly in any language.

Answer 63

An internal data structure that is used to represent the result set of a query, or other operation that performs a search using an SQL WHERE clause. It works like an iterator in other high-level languages, producing each value from the result set as requested. Although usually SQL handles the processing of cursors for you, you might delve into the inner workings when dealing with performance-critical code.

Answer 64

Metadata that keeps track of InnoDB-related objects such as tables, indexes, and table columns. This metadata is physically located in the InnoDB system tablespace. For historical reasons, it overlaps to some degree with information stored in the .frm files. Because the MySQL Enterprise Backup product always backs up the system tablespace, all backups include the contents of the data dictionary.

Answer 65

The directory under which each MySQL instance keeps the data files for InnoDB and the directories representing individual databases. Controlled by the datadir configuration option.

Answer 66

The files that physically contain the InnoDB table and index data. There can be a one-to-many relationship between data files and tables, as in the case of the system tablespace, which can hold multiple InnoDB tables as well as the data dictionary. There can also be a one-to-one relationship between data files and tables, as when the file-per-table setting is enabled, causing each newly created table to be stored in a separate tablespace.

Answer 67

A database system or application that primarily runs large queries. The read-only or read-mostly data might be organized in denormalized form for query efficiency. Can benefit from the optimizations for read-only transactions in MySQL 5.6 and higher. Contrast with OLTP.

Answer 68

Within the MySQL data directory, each database is represented by a separate directory. The InnoDB system tablespace, which can hold table data from multiple databases within a MySQL instance, is kept in its data files that reside outside the individual database directories. When file-per-table mode is enabled, the .ibd files representing individual InnoDB tables are stored inside the database directories. For long-time MySQL users, a database is a familiar notion. Users coming from an Oracle Database background will find that the MySQL meaning of a database is closer to what Oracle Database calls a schema.

Answer 69

Data control language, a set of SQL statements for managing privileges. In MySQL, consists of the GRANT and REVOKE statements. Contrast with DDL and DML.

Answer 70

Data definition language, a set of SQL statements for manipulating the database itself rather than individual table rows. Includes all forms of the CREATE, ALTER, and DROP statements. Also includes the TRUNCATE statement, because it works differently than a DELETE FROM table\_name statement, even though the ultimate effect is similar. DDL statements automatically commit the current transaction; they cannot be rolled back. The InnoDB online DDL feature enhances performance for CREATE INDEX, DROP INDEX, and many types of ALTER TABLE operations. See Section 15.12, “InnoDB and Online DDL” for more information. Also, the InnoDB file-per-table setting can affect the behavior of DROP TABLE and TRUNCATE TABLE operations. Contrast with DML and DCL.

Answer 71

A situation where different transactions are unable to proceed, because each holds a lock that the other needs. Because both transactions are waiting for a resource to become available, neither will ever release the locks it holds. A deadlock can occur when the transactions lock rows in multiple tables (through statements such as UPDATE or SELECT ... FOR UPDATE), but in the opposite order. A deadlock can also occur when such statements lock ranges of index records and gaps, with each transaction acquiring some locks but not others due to a timing issue. To reduce the possibility of deadlocks, use transactions rather than LOCK TABLE statements; keep transactions that insert or update data small enough that they do not stay open for long periods of time; when different transactions update multiple tables or large ranges of rows, use the same order of operations (such as SELECT ... FOR UPDATE) in each transaction; create indexes on the columns used in SELECT ... FOR UPDATE and UPDATE ... WHERE statements. The possibility of deadlocks is not affected by the isolation level, because the isolation level changes the behavior of read operations, while deadlocks occur because of write operations. If a deadlock does occur, InnoDB detects the condition and rolls back one of the transactions (the victim). Thus, even if your application logic is perfectly correct, you must still handle the case where a transaction must be retried. To see the last deadlock in an InnoDB user transaction, use the command SHOW ENGINE INNODB STATUS. If frequent deadlocks highlight a problem with transaction structure or application error handling, run with the innodb\_print\_all\_deadlocks setting enabled to print information about all deadlocks to the mysqld error log. For background information on how deadlocks are automatically detected and handled, see Section 14.2.10, “Deadlock Detection and Rollback”. For tips on avoiding and recovering from deadlock conditions, see Section 14.2.11, “How to Cope with Deadlocks”.

Answer 72

A mechanism that automatically detects when a deadlock occurs, and automatically rolls back one of the transactions involved (the victim).

Answer 73

When InnoDB processes a DELETE statement, the rows are immediately marked for deletion and no longer are returned by queries. The storage is reclaimed sometime later, during the periodic garbage collection known as the purge operation, performed by a separate thread. For removing large quantities of data, related operations with their own performance characteristics are truncate and drop.

Answer 74

The technique of storing index changes due to DELETE operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. (Because delete operations are a two-step process, this operation buffers the write that normally marks an index record for deletion.) It is one of the types of change buffering; the others are insert buffering and purge buffering.

Answer 75

A data storage strategy that duplicates data across different tables, rather than linking the tables with foreign keys and join queries. Typically used in data warehouse applications, where the data is not updated after loading. In such applications, query performance is more important than making it simple to maintain consistent data during updates. Contrast with normalized.

Answer 76

A type of index available with some database systems, where index storage is optimized to process ORDER BY column DESC clauses. Currently, although MySQL allows the DESC keyword in the CREATE TABLE statement, it does not use any special storage layout for the resulting index.

Answer 77

A page in the InnoDB buffer pool that has been updated in memory, where the changes are not yet written (flushed) to the data files. The opposite of a clean page.

Answer 78

A type of workload where the primary bottleneck is disk I/O. (Also known as I/O-bound.) Typically involves frequent writes to disk, or random reads of more data than can fit into the buffer pool.

Answer 79

Data manipulation language, a set of SQL statements for performing insert, update, and delete operations. The SELECT statement is sometimes considered as a DML statement, because the SELECT ... FOR UPDATE form is subject to the same considerations for locking as INSERT, UPDATE, and DELETE. DML statements for an InnoDB table operate in the context of a transaction, so their effects can be committed or rolled back as a single unit. Contrast with DDL and DCL.

Answer 80

In the InnoDB full-text search feature, a special column in the table containing the FULLTEXT index, to uniquely identify the document associated with each ilist value. Its name is FTS\_DOC\_ID (uppercase required). The column itself must be of BIGINT UNSIGNED NOT NULL type, with a unique index named FTS\_DOC\_ID\_INDEX. Preferably, you define this column when creating the table. If InnoDB must add the column to the table while creating a FULLTEXT index, the indexing operation is considerably more expensive.

Answer 81

InnoDB uses a novel file flush technique called doublewrite. Before writing pages to the data files, InnoDB first writes them to a contiguous area called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer have completed, does InnoDB write the pages to their proper positions in the data file. If there is an operating system, storage subsystem, or mysqld process crash in the middle of a page write, InnoDB can later find a good copy of the page from the doublewrite buffer during crash recovery. Although data is always written twice, the doublewrite buffer does not require twice as much I/O overhead or twice as many I/O operations. Data is written to the buffer itself as a large sequential chunk, with a single fsync() call to the operating system. To turn off the doublewrite buffer, specify the option innodb\_doublewrite=0.

Answer 82

A kind of DDL operation that removes a schema object, through a statement such as DROP TABLE or DROP INDEX. It maps internally to an ALTER TABLE statement. From an InnoDB perspective, the performance considerations of such operations involve the time that the data dictionary is locked to ensure that interrelated objects are all updated, and the time to update memory structures such as the buffer pool. For atable, the drop operation has somewhat different characteristics than a truncate operation (TRUNCATE TABLE statement).

Answer 83

A row format introduced in the InnoDB Plugin, available as part of the Barracuda file format. Because TEXT and BLOB fields are stored outside of the rest of the page that holds the row data, it is very efficient for rows that include large objects. Since the large fields are typically not accessed to evaluate query conditions, they are not brought into the buffer pool as often, resulting in fewer I/O operations and better utilization of cache memory. For additional information about InnoDB DYNAMIC row format, see Section 14.9.3, “DYNAMIC and COMPRESSED Row Formats”.

Answer 84

A stage similar to beta, when a software product is typically evaluated for performance, functionality, and compatibility in a non-mission-critical setting. InnoDB uses the early adopter designation rather than beta, through a succession of point releases leading up to a GA release.

Answer 85

A type of log showing information about MySQL startup and critical runtime errors and crash information. For details, see Section 5.2.2, “The Error Log”.

Answer 86

The process of removing an item from a cache or other temporary storage area, such as the InnoDB buffer pool. Often, but not always, uses theLRU algorithm to determine which item to remove. When a dirty page is evicted, its contents are flushed to disk, and any dirty neighbor pages might be flushed also.

Answer 87

A kind of lock that prevents any other transaction from locking the same row. Depending on the transaction isolation level, this kind of lock might block other transactions from writing to the same row, or might also block other transactions from reading the same row. The default InnoDB isolation level, REPEATABLE READ, enables higher concurrency by allowing transactions to read rows that have exclusive locks, a technique known as consistent read.

Answer 88

A group of pages within a tablespace totaling 1 megabyte (or 1048576 bytes). With the default page size of 16KB, an extent contains 64 pages. In MySQL 5.6, the page size for an InnoDB instance can be 4KB, 8KB, or 16KB, controlled by the innodb\_page\_size configuration option. If a 4KB or 8KB page size is used, the extent contains more pages but is still 1MB in size. InnoDB features such as segments, read-ahead requests and the doublewrite buffer use I/O operations that read, write, allocate, or free data one extent at a time.

Answer 89

The default shutdown procedure for InnoDB, based on the configuration setting innodb\_fast\_shutdown=1. To save time, certainflush operations are skipped. This type of shutdown is safe during normal usage, because the flush operations are performed during the next startup, using the same mechanism as in crash recovery. In cases where the database is being shut down for an upgrade or downgrade, do aslow shutdown instead to ensure that all relevant changes are applied to the data files during the shutdown.

Answer 90

The format used by InnoDB for each table, typically with the file-per-table setting enabled so that each table is stored in a separate .ibd file. Currently, the file formats available in InnoDB are known as Antelope and Barracuda. Each file format supports one or more row formats. The row formats available for Barracuda tables, COMPRESSED and DYNAMIC, enable important new storage features for InnoDB tables.

Answer 91

A general name for the setting controlled by the innodb\_file\_per\_table option. That is a very important configuration option that affects many aspects of InnoDB file storage, availability of features, and I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is disabled by default. For each table created while this setting is in effect, the data is stored in a separate .ibd file rather than in the ibdata files of the system tablespace. When table data is stored in individual files, you have more flexibility to choose nondefault file formats and row formats, which are required for features such as data compression. The TRUNCATE TABLE operation is also much faster, and the reclaimed space can be used by the operating system rather than remaining reserved for InnoDB. The MySQL Enterprise Backup product is more flexible for tables that are in their own files. For example, tables can be excluded from a backup, but only if they are in separate files. Thus, this setting is suitable for tables that are backed up less frequently or on a different schedule.

Answer 92

In an InnoDB index, the proportion of a page that is taken up by index data before the page is split. The unused space when index data is first divided between pages allows for rows to be updated with longer string values without requiring expensive index maintenance operations. If the fill factor is too low, the index consumes more space than needed, causing extra I/O overhead when reading the index. If the fill factor is too high, any update that increases the length of column values can cause extra I/O overhead for index maintenance. See Section 14.2.13.4, “Physical Structure of an InnoDB Index” for more information.

Answer 93

This row format is used by the MyISAM storage engine, not by InnoDB. If you create an InnoDB table with the option row\_format=fixed, InnoDB translates this option to use the compact row format instead, although the fixed value might still show up in output such as SHOW TABLE STATUS reports.

Answer 94

To write changes to the database files, that had been buffered in a memory area or a temporary disk storage area. The InnoDB storage structures that are periodically flushed include the redo log, the undo log, and the buffer pool. Flushing can happen because a memory area becomes full and the system needs to free some space, because a commit operation means the changes from a transaction can be finalized, or because a slow shutdown operation means that all outstanding work should be finalized. When it is not critical to flush all the buffered data at once, InnoDB can use a technique called fuzzy checkpointing to flush small batches of pages to spread out the I/O overhead.

Answer 95

An internal InnoDB data structure that tracks dirty pages in the buffer pool: that is, pages that have been changed and need to be written back out to disk. This data structure is updated frequently by InnoDB's internal mini-transactions, and so is protected by its own mutex to allow concurrent access to the buffer pool.

Answer 96

A type of pointer relationship, between rows in separate InnoDB tables. The foreign key relationship is defined on one column in both the parent table and the child table. In addition to enabling fast lookup of related information, foreign keys help to enforce referential integrity, by preventing any of these pointers from becoming invalid as data is inserted, updated, and deleted. This enforcement mechanism is a type of constraint. A row that points to another table cannot be inserted if the associated foreign key value does not exist in the other table. If a row is deleted or its foreign key value changed, and rows in another table point to that foreign key value, the foreign key can be set up to prevent the deletion, cause the corresponding column values in the other table to become null, or automatically delete the corresponding rows in the other table. One of the stages in designing a normalized database is to identify data that is duplicated, separate that data into a new table, and set up a foreign key relationship so that the multiple tables can be queried like a single table, using a join operation.

Answer 97

The type of constraint that maintains database consistency through a foreign key relationship. Like other kinds of constraints, it can prevent data from being inserted or updated if data would become inconsistent; in this case, the inconsistency being prevented is between data in multiple tables. Alternatively, when a DML operation is performed, FOREIGN KEY constraints can cause data in child rows to be deleted, changed to different values, or set to null, based on the ON CASCADE option specified when creating the foreign key.

Answer 98

In most contexts, an acronym for full-text search. Sometimes in performance discussions, an acronym for full table scan.

Answer 99

A backup that includes all the tables in each MySQL database, and all the databases in a MySQL instance. Contrast with partial backup.

Answer 100

An operation that requires reading the entire contents of a table, rather than just selected portions using an index. Typically performed either with small lookup tables, or in data warehousing situations with large tables where all available data is aggregated and analyzed. How frequently these operations occur, and the sizes of the tables relative to available memory, have implications for the algorithms used in query optimization and managing the buffer pool. The purpose of indexes is to allow lookups for specific values or ranges of values within a large table, thus avoiding full table scans when practical.

Answer 101

The MySQL feature for finding words, phrases, Boolean combinations of words, and so on within table data, in a faster, more convenient, and more flexible way than using the SQL LIKE operator or writing your own application-level search algorithm. It uses the SQL function MATCH()and FULLTEXT indexes.

Answer 102

The special kind of index that holds the search index in the MySQL full-text search mechanism. Represents the words from values of a column, omitting any that are specified as stopwords. Originally, only available for MyISAM tables. Starting in MySQL 5.6.4, it is also available for InnoDB tables.

Answer 103

A technique that flushes small batches of dirty pages from the buffer pool, rather than flushing all dirty pages at once which would disrupt database processing.

Answer 104

"Generally available", the stage when a software product leaves beta and is available for sale, official support, and production use.

Answer 105

A place in an InnoDB index data structure where new values could be inserted. When you lock a set of rows with a statement such as SELECT ... FOR UPDATE, InnoDB can create locks that apply to the gaps as well as the actual values in the index. For example, if you select all values greater than 10 for update, a gap lock prevents another transaction from inserting a new value that is greater than 10. The supremum record and infimum record represent the gaps containing all values greater than or less than all the current index values.

Answer 106

A lock on a gap between index records, or a lock on the gap before the first or after the last index record. For example, SELECT c1 FOR UPDATE FROM t WHERE c1 BETWEEN 10 and 20; prevents other transactions from inserting a value of 15 into the column t.c1, whether or not there was already any such value in the column, because the gaps between all existing values in the range are locked. Contrast with record lock and next-key lock. Gap locks are part of the tradeoff between performance and concurrency, and are used in some transaction isolation levels and not others.

Answer 107

A type of log used for diagnosis and troubleshooting of SQL statements processed by the MySQL server. Can be stored in a file or in a database table. You must enable this feature through the general\_log configuration option to use it. You can disable it for a specific connection through the sql\_log\_off configuration option. Records a broader range of queries than the slow query log. Unlike the binary log, which is used for replication, the general query log contains SELECT statements and does not maintain strict ordering. For more information, see Section 5.2.3, “The General Query Log”.

Answer 108

A type of transaction involved in XA operations. It consists of several actions that are transactional in themselves, but that all must either complete successfully as a group, or all be rolled back as a group. In essence, this extends ACID properties "up a level" so that multiple ACID transactions can be executed in concert as components of a global operation that also has ACID properties. For this type of distributed transaction, you must use the SERIALIZABLE isolation level to achieve ACID properties.

Answer 109

An InnoDB optimization that performs some low-level I/O operations (log write) once for a set of commit operations, rather than flushing and syncing separately for each commit. When the binary log is enabled, you typically also set the configuration option sync\_binlog=0, because group commit for the binary log is only supported if it is set to 0.

Answer 110

A type of index intended for queries that use equality operators, rather than range operators such as greater-than or BETWEEN. It is available for MEMORY tables. Although hash indexes are the default for MEMORY tables for historic reasons, that storage engine also supports B-tree indexes, which are often a better choice for general-purpose queries. MySQL includes a variant of this index type, the adaptive hash index, that is constructed automatically for InnoDB tables if needed based on runtime conditions.

Answer 111

Acronym for "hard disk drive". Refers to storage media using spinning platters, usually when comparing and contrasting with SSD. Its performance characteristics can influence the throughput of a disk-based workload.

Answer 112

A periodic message that is sent to indicate that a system is functioning properly. In a replication context, if the master stops sending such messages, one of the slaves can take its place. Similar techniques can be used between the servers in a cluster environment, to confirm that all of them are operating properly.

Answer 113

A value representing an upper limit, either a hard limit that should not be exceeded at runtime, or a record of the maximum value that was actually reached. Contrast with low-water mark.

Answer 114

A list of transactions with delete-marked records scheduled to be processed by the InnoDB purge operation. Recorded in the undo log. The length of the history list is reported by the command SHOW ENGINE INNODB STATUS. If the history list grows longer than the value of the innodb\_max\_purge\_lag configuration option, each DML operation is delayed slightly to allow the purge operation to finish flushing the deleted records. Also known as purge lag.

Answer 115

A condition where a row, table, or internal data structure is accessed so frequently, requiring some form of locking or mutual exclusion, that it results in a performance or scalability issue. Although "hot" typically indicates an undesirable condition, a hot backup is the preferred type of backup.

Answer 116

A backup taken while the database and is running and applications are reading and writing to it. The backup involves more than simply copying data files: it must include any data that was inserted or updated while the backup was in process; it must exclude any data that was deleted while the backup was in process; and it must ignore any changes that were not committed. The Oracle product that performs hot backups, of InnoDB tables especially but also tables from MyISAM and other storage engines, is known as MySQL Enterprise Backup. The hot backup process consists of two stages. The initial copying of the data files produces a raw backup. The apply step incorporates any changes to the database that happened while the backup was running. Applying the changes produces a prepared backup; these files are ready to be restored whenever necessary.

Answer 117

The set of files managed by InnoDB within a MySQL database: the system tablespace, any file-per-table tablespaces, and the (typically 2)redo log files. Used sometimes in detailed discussions of InnoDB file structures and formats, to avoid ambiguity between the meanings ofdatabase between different DBMS products, and the non-InnoDB files that may be part of a MySQL database.

Answer 118

A supplemental backup file created by the MySQL Enterprise Backup product during a hot backup operation. It contains information about any data changes that occurred while the backup was running. The initial backup files, including ibbackup\_logfile, are known as a raw backup, because the changes that occurred during the backup operation are not yet incorporated. After you perform the apply step to the raw backup files, the resulting files do include those final data changes, and are known as a prepared backup. At this stage, theibbackup\_logfile file is no longer necessary.

Answer 119

A set of files with names such as ibdata1, ibdata2, and so on, that make up the InnoDB system tablespace. These files contain metadata about InnoDB tables, (the data dictionary), and the storage areas for the undo log, the change buffer, and the doublewrite buffer. They also can contain some or all of the table data also (depending on whether the file-per-table mode is in effect when each table is created). When the innodb\_file\_per\_table option is enabled, data and indexes for newly created tables are stored in separate .ibd files rather than in the system tablespace. The growth of the ibdata files is influenced by the innodb\_autoextend\_increment configuration option.

Answer 120

The InnoDB temporary tablespace data file for non-compressed InnoDB temporary tables and related objects. The configuration file option,innodb\_temp\_data\_file\_path, allows users to define a relative path for the temporary data file. Ifinnodb\_temp\_data\_file\_path is not specified, the default behavior is to create a single auto-extending 12MB data file namedibtmp1 in the data directory, alongside ibdata1.

Answer 121

A set of files, typically named ib\_logfile0 and ib\_logfile1, that form the redo log. Also sometimes referred to as the log group. These files record statements that attempt to change data in InnoDB tables. These statements are replayed automatically to correct data written by incomplete transactions, on startup following a crash. This data cannot be used for manual recovery; for that type of operation, use the binary log.

Answer 122

Within an InnoDB FULLTEXT index, the data structure consisting of a document ID and positional information for a token (that is, a particular word).

Answer 123

A row lock that InnoDB acquires to ensure consistency, without you specifically requesting it.

Answer 124

A row lock that InnoDB acquires to ensure consistency, without you specifically requesting it.

Answer 125

A row lock that InnoDB acquires to ensure consistency, without you specifically requesting it.

Answer 126

A type of database system that maintains data in memory, to avoid overhead due to disk I/O and translation between disk blocks and memory areas. Some in-memory databases sacrifice durability (the "D" in the ACID design philosophy) and are vulnerable to hardware, power, and other types of failures, making them more suitable for read-only operations. Other in-memory databases do use durability mechanisms such as logging changes to disk or using non-volatile memory. MySQL features that are address the same kinds of memory-intensive processing include the InnoDB buffer pool, adaptive hash index, and read-only transaction optimization, the MEMORY storage engine, the MyISAM key cache, and the MySQL query cache.

Answer 127

A type of database system that maintains data in memory, to avoid overhead due to disk I/O and translation between disk blocks and memory areas. Some in-memory databases sacrifice durability (the "D" in the ACID design philosophy) and are vulnerable to hardware, power, and other types of failures, making them more suitable for read-only operations. Other in-memory databases do use durability mechanisms such as logging changes to disk or using non-volatile memory. MySQL features that are address the same kinds of memory-intensive processing include the InnoDB buffer pool, adaptive hash index, and read-only transaction optimization, the MEMORY storage engine, the MyISAM key cache, and the MySQL query cache.

Answer 128

A type of database system that maintains data in memory, to avoid overhead due to disk I/O and translation between disk blocks and memory areas. Some in-memory databases sacrifice durability (the "D" in the ACID design philosophy) and are vulnerable to hardware, power, and other types of failures, making them more suitable for read-only operations. Other in-memory databases do use durability mechanisms such as logging changes to disk or using non-volatile memory. MySQL features that are address the same kinds of memory-intensive processing include the InnoDB buffer pool, adaptive hash index, and read-only transaction optimization, the MEMORY storage engine, the MyISAM key cache, and the MySQL query cache.

Answer 129

A type of hot backup, performed by the MySQL Enterprise Backup product, that only saves data changed since some point in time. Having a full backup and a succession of incremental backups lets you reconstruct backup data over a long period, without the storage overhead of keeping several full backups on hand. You can restore the full backup and then apply each of the incremental backups in succession, or you can keep the full backup up-to-date by applying each incremental backup to it, then perform a single restore operation. The granularity of changed data is at the page level. A page might actually cover more than one row. Each changed page is included in the backup.

Answer 130

A type of hot backup, performed by the MySQL Enterprise Backup product, that only saves data changed since some point in time. Having a full backup and a succession of incremental backups lets you reconstruct backup data over a long period, without the storage overhead of keeping several full backups on hand. You can restore the full backup and then apply each of the incremental backups in succession, or you can keep the full backup up-to-date by applying each incremental backup to it, then perform a single restore operation. The granularity of changed data is at the page level. A page might actually cover more than one row. Each changed page is included in the backup.

Answer 131

A type of hot backup, performed by the MySQL Enterprise Backup product, that only saves data changed since some point in time. Having a full backup and a succession of incremental backups lets you reconstruct backup data over a long period, without the storage overhead of keeping several full backups on hand. You can restore the full backup and then apply each of the incremental backups in succession, or you can keep the full backup up-to-date by applying each incremental backup to it, then perform a single restore operation. The granularity of changed data is at the page level. A page might actually cover more than one row. Each changed page is included in the backup.

Answer 132

A data structure that provides a fast lookup capability for rows of a table, typically by forming a tree structure (B-tree) representing all the values of a particular column or set of columns. InnoDB tables always have a clustered index representing the primary key. They can also have one or more secondary indexes defined on one or more columns. Depending on their structure, secondary indexes can be classified as partial, column, or composite indexes. Indexes are a crucial aspect of query performance. Database architects design tables, queries, and indexes to allow fast lookups for data needed by applications. The ideal database design uses a covering index where practical; the query results are computed entirely from the index, without reading the actual table data. Each foreign key constraint also requires an index, to efficiently check whether values exist in both the parent and child tables. Although a B-tree index is the most common, a different kind of data structure is used for hash indexes, as in the MEMORY storage engine and the InnoDB adaptive hash index.

Answer 133

A data structure that provides a fast lookup capability for rows of a table, typically by forming a tree structure (B-tree) representing all the values of a particular column or set of columns. InnoDB tables always have a clustered index representing the primary key. They can also have one or more secondary indexes defined on one or more columns. Depending on their structure, secondary indexes can be classified as partial, column, or composite indexes. Indexes are a crucial aspect of query performance. Database architects design tables, queries, and indexes to allow fast lookups for data needed by applications. The ideal database design uses a covering index where practical; the query results are computed entirely from the index, without reading the actual table data. Each foreign key constraint also requires an index, to efficiently check whether values exist in both the parent and child tables. Although a B-tree index is the most common, a different kind of data structure is used for hash indexes, as in the MEMORY storage engine and the InnoDB adaptive hash index.

Answer 134

A data structure that provides a fast lookup capability for rows of a table, typically by forming a tree structure (B-tree) representing all the values of a particular column or set of columns. InnoDB tables always have a clustered index representing the primary key. They can also have one or more secondary indexes defined on one or more columns. Depending on their structure, secondary indexes can be classified as partial, column, or composite indexes. Indexes are a crucial aspect of query performance. Database architects design tables, queries, and indexes to allow fast lookups for data needed by applications. The ideal database design uses a covering index where practical; the query results are computed entirely from the index, without reading the actual table data. Each foreign key constraint also requires an index, to efficiently check whether values exist in both the parent and child tables. Although a B-tree index is the most common, a different kind of data structure is used for hash indexes, as in the MEMORY storage engine and the InnoDB adaptive hash index.

Answer 135

A memory area that holds the token data for InnoDB full-text search. It buffers the data to minimize disk I/O when data is inserted or updated in columns that are part of a FULLTEXT index. The token data is written to disk when the index cache becomes full. Each InnoDB FULLTEXTindex has its own separate index cache, whose size is controlled by the configuration option innodb\_ft\_cache\_size.

Answer 136

A memory area that holds the token data for InnoDB full-text search. It buffers the data to minimize disk I/O when data is inserted or updated in columns that are part of a FULLTEXT index. The token data is written to disk when the index cache becomes full. Each InnoDB FULLTEXTindex has its own separate index cache, whose size is controlled by the configuration option innodb\_ft\_cache\_size.

Answer 137

A memory area that holds the token data for InnoDB full-text search. It buffers the data to minimize disk I/O when data is inserted or updated in columns that are part of a FULLTEXT index. The token data is written to disk when the index cache becomes full. Each InnoDB FULLTEXTindex has its own separate index cache, whose size is controlled by the configuration option innodb\_ft\_cache\_size.

Answer 138

Extended SQL syntax for overriding the indexes recommended by the optimizer. For example, the FORCE INDEX, USE INDEX, andIGNORE INDEX clauses. Typically used when indexed columns have unevenly distributed values, resulting in inaccurate cardinalityestimates.

Answer 139

Extended SQL syntax for overriding the indexes recommended by the optimizer. For example, the FORCE INDEX, USE INDEX, andIGNORE INDEX clauses. Typically used when indexed columns have unevenly distributed values, resulting in inaccurate cardinalityestimates.

Answer 140

Extended SQL syntax for overriding the indexes recommended by the optimizer. For example, the FORCE INDEX, USE INDEX, andIGNORE INDEX clauses. Typically used when indexed columns have unevenly distributed values, resulting in inaccurate cardinalityestimates.

Answer 141

In an index that applies to multiple columns (known as a composite index), the initial or leading columns of the index. A query that references the first 1, 2, 3, and so on columns of a composite index can use the index, even if the query does not reference all the columns in the index.

Answer 142

In an index that applies to multiple columns (known as a composite index), the initial or leading columns of the index. A query that references the first 1, 2, 3, and so on columns of a composite index can use the index, even if the query does not reference all the columns in the index.

Answer 143

In an index that applies to multiple columns (known as a composite index), the initial or leading columns of the index. A query that references the first 1, 2, 3, and so on columns of a composite index can use the index, even if the query does not reference all the columns in the index.

Answer 144

A pseudo-record in an index, representing the gap below the smallest value in that index. If a transaction has a statement such as SELECT ... FOR UPDATE ... WHERE col \< 10;, and the smallest value in the column is 5, it is a lock on the infimum record that prevents other transactions from inserting even smaller values such as 0, -10, and so on.

Answer 145

A pseudo-record in an index, representing the gap below the smallest value in that index. If a transaction has a statement such as SELECT ... FOR UPDATE ... WHERE col \< 10;, and the smallest value in the column is 5, it is a lock on the infimum record that prevents other transactions from inserting even smaller values such as 0, -10, and so on.

Answer 146

A pseudo-record in an index, representing the gap below the smallest value in that index. If a transaction has a statement such as SELECT ... FOR UPDATE ... WHERE col \< 10;, and the smallest value in the column is 5, it is a lock on the infimum record that prevents other transactions from inserting even smaller values such as 0, -10, and so on.

Answer 147

The name of the database that provides a query interface to the MySQL data dictionary. (This name is defined by the ANSI SQL standard.) To examine information (metadata) about the database, you can query tables such as INFORMATION\_SCHEMA.TABLES and INFORMATION\_SCHEMA.COLUMNS, rather than using SHOW commands that produce unstructured output. The information schema contains some tables that are specific to InnoDB, such as INNODB\_LOCKS and INNODB\_TRX. You use these tables not to see how the database is structured, but to get real-time information about the workings of InnoDB tables to help with performance monitoring, tuning, and troubleshooting. In particular, these tables provide information about MySQL features related to compression, and transactions and their associated locks.

Answer 148

The name of the database that provides a query interface to the MySQL data dictionary. (This name is defined by the ANSI SQL standard.) To examine information (metadata) about the database, you can query tables such as INFORMATION\_SCHEMA.TABLES and INFORMATION\_SCHEMA.COLUMNS, rather than using SHOW commands that produce unstructured output. The information schema contains some tables that are specific to InnoDB, such as INNODB\_LOCKS and INNODB\_TRX. You use these tables not to see how the database is structured, but to get real-time information about the workings of InnoDB tables to help with performance monitoring, tuning, and troubleshooting. In particular, these tables provide information about MySQL features related to compression, and transactions and their associated locks.

Answer 149

The name of the database that provides a query interface to the MySQL data dictionary. (This name is defined by the ANSI SQL standard.) To examine information (metadata) about the database, you can query tables such as INFORMATION\_SCHEMA.TABLES and INFORMATION\_SCHEMA.COLUMNS, rather than using SHOW commands that produce unstructured output. The information schema contains some tables that are specific to InnoDB, such as INNODB\_LOCKS and INNODB\_TRX. You use these tables not to see how the database is structured, but to get real-time information about the workings of InnoDB tables to help with performance monitoring, tuning, and troubleshooting. In particular, these tables provide information about MySQL features related to compression, and transactions and their associated locks.

Answer 150

A MySQL component that combines high performance with transactional capability for reliability, robustness, and concurrent access. It embodies the ACID design philosophy. Represented as a storage engine; it handles tables created or altered with the ENGINE=INNODB clause. See Chapter 14, The InnoDB Storage Engine for architectural details and administration procedures, and Section 8.5, “Optimizing for InnoDB Tables” for performance advice. In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables and the ENGINE=INNODB clause is not required. In MySQL 5.1 only, many of the advanced InnoDB features require enabling the component known as the InnoDB Plugin. See Section 14.1.1, “InnoDB as the Default MySQL Storage Engine” for the considerations involved in transitioning to recent releases where InnoDB tables are the default. InnoDB tables are ideally suited for hot backups. See Section 25.2, “MySQL Enterprise Backup” for information about the MySQL Enterprise Backup product for backing up MySQL servers without interrupting normal processing.

Answer 151

A MySQL component that combines high performance with transactional capability for reliability, robustness, and concurrent access. It embodies the ACID design philosophy. Represented as a storage engine; it handles tables created or altered with the ENGINE=INNODB clause. See Chapter 14, The InnoDB Storage Engine for architectural details and administration procedures, and Section 8.5, “Optimizing for InnoDB Tables” for performance advice. In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables and the ENGINE=INNODB clause is not required. In MySQL 5.1 only, many of the advanced InnoDB features require enabling the component known as the InnoDB Plugin. See Section 14.1.1, “InnoDB as the Default MySQL Storage Engine” for the considerations involved in transitioning to recent releases where InnoDB tables are the default. InnoDB tables are ideally suited for hot backups. See Section 25.2, “MySQL Enterprise Backup” for information about the MySQL Enterprise Backup product for backing up MySQL servers without interrupting normal processing.

Answer 152

A MySQL component that combines high performance with transactional capability for reliability, robustness, and concurrent access. It embodies the ACID design philosophy. Represented as a storage engine; it handles tables created or altered with the ENGINE=INNODB clause. See Chapter 14, The InnoDB Storage Engine for architectural details and administration procedures, and Section 8.5, “Optimizing for InnoDB Tables” for performance advice. In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables and the ENGINE=INNODB clause is not required. In MySQL 5.1 only, many of the advanced InnoDB features require enabling the component known as the InnoDB Plugin. See Section 14.1.1, “InnoDB as the Default MySQL Storage Engine” for the considerations involved in transitioning to recent releases where InnoDB tables are the default. InnoDB tables are ideally suited for hot backups. See Section 25.2, “MySQL Enterprise Backup” for information about the MySQL Enterprise Backup product for backing up MySQL servers without interrupting normal processing.

Answer 153

The innodb\_autoinc\_lock\_mode option controls the algorithm used for auto-increment locking. When you have an auto-incrementing primary key, you can use statement-based replication only with the setting innodb\_autoinc\_lock\_mode=1. This setting is known as consecutive lock mode, because multi-row inserts within a transaction receive consecutive auto-increment values. If you have innodb\_autoinc\_lock\_mode=2, which allows higher concurrency for insert operations, use row-based replication rather than statement-based replication. This setting is known as interleaved lock mode, because multiple multi-row insert statements running at the same time can receive autoincrement values that are interleaved. The setting innodb\_autoinc\_lock\_mode=0 is the previous (traditional) default setting and should not be used except for compatibility purposes.

Answer 154

The innodb\_autoinc\_lock\_mode option controls the algorithm used for auto-increment locking. When you have an auto-incrementing primary key, you can use statement-based replication only with the setting innodb\_autoinc\_lock\_mode=1. This setting is known as consecutive lock mode, because multi-row inserts within a transaction receive consecutive auto-increment values. If you have innodb\_autoinc\_lock\_mode=2, which allows higher concurrency for insert operations, use row-based replication rather than statement-based replication. This setting is known as interleaved lock mode, because multiple multi-row insert statements running at the same time can receive autoincrement values that are interleaved. The setting innodb\_autoinc\_lock\_mode=0 is the previous (traditional) default setting and should not be used except for compatibility purposes.

Answer 155

The innodb\_autoinc\_lock\_mode option controls the algorithm used for auto-increment locking. When you have an auto-incrementing primary key, you can use statement-based replication only with the setting innodb\_autoinc\_lock\_mode=1. This setting is known as consecutive lock mode, because multi-row inserts within a transaction receive consecutive auto-increment values. If you have innodb\_autoinc\_lock\_mode=2, which allows higher concurrency for insert operations, use row-based replication rather than statement-based replication. This setting is known as interleaved lock mode, because multiple multi-row insert statements running at the same time can receive autoincrement values that are interleaved. The setting innodb\_autoinc\_lock\_mode=0 is the previous (traditional) default setting and should not be used except for compatibility purposes.

Answer 156

The innodb\_file\_format option determines the file format for all InnoDB tablespaces created after you specify a value for this option. To create tablespaces other than the system tablespace, you must also use the file-per-table option. Currently, you can specify the Antelopeand Barracuda file formats.

Answer 157

The innodb\_file\_format option determines the file format for all InnoDB tablespaces created after you specify a value for this option. To create tablespaces other than the system tablespace, you must also use the file-per-table option. Currently, you can specify the Antelopeand Barracuda file formats.

Answer 158

The innodb\_file\_format option determines the file format for all InnoDB tablespaces created after you specify a value for this option. To create tablespaces other than the system tablespace, you must also use the file-per-table option. Currently, you can specify the Antelopeand Barracuda file formats.

Answer 159

A very important configuration option that affects many aspects of InnoDB file storage, availability of features, and I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is disabled by default. The innodb\_file\_per\_table option turns on file-per-table mode, which stores each newly created InnoDB table and its associated index in its own .ibd file, outside the system tablespace. This option affects the performance and storage considerations for a number of SQL statements, such as DROP TABLE and TRUNCATE TABLE. This option is needed to take full advantage of many other InnoDB features, such as such as table compression, or backups of named tables in MySQL Enterprise Backup. This option was once static, but can now be set using the SET GLOBAL command. For reference information, see innodb\_file\_per\_table. For usage information, see Section 14.5.2, “InnoDB File-Per-Table Mode”.

Answer 160

A very important configuration option that affects many aspects of InnoDB file storage, availability of features, and I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is disabled by default. The innodb\_file\_per\_table option turns on file-per-table mode, which stores each newly created InnoDB table and its associated index in its own .ibd file, outside the system tablespace. This option affects the performance and storage considerations for a number of SQL statements, such as DROP TABLE and TRUNCATE TABLE. This option is needed to take full advantage of many other InnoDB features, such as such as table compression, or backups of named tables in MySQL Enterprise Backup. This option was once static, but can now be set using the SET GLOBAL command. For reference information, see innodb\_file\_per\_table. For usage information, see Section 14.5.2, “InnoDB File-Per-Table Mode”.

Answer 161

A very important configuration option that affects many aspects of InnoDB file storage, availability of features, and I/O characteristics. In MySQL 5.6.7 and higher, it is enabled by default. Prior to MySQL 5.6.7, it is disabled by default. The innodb\_file\_per\_table option turns on file-per-table mode, which stores each newly created InnoDB table and its associated index in its own .ibd file, outside the system tablespace. This option affects the performance and storage considerations for a number of SQL statements, such as DROP TABLE and TRUNCATE TABLE. This option is needed to take full advantage of many other InnoDB features, such as such as table compression, or backups of named tables in MySQL Enterprise Backup. This option was once static, but can now be set using the SET GLOBAL command. For reference information, see innodb\_file\_per\_table. For usage information, see Section 14.5.2, “InnoDB File-Per-Table Mode”.

Answer 162

The innodb\_lock\_wait\_timeout option sets the balance between waiting for shared resources to become available, or giving up and handling the error, retrying, or doing alternative processing in your application. Rolls back any InnoDB transaction that waits more than a specified time to acquire a lock. Especially useful if deadlocks are caused by updates to multiple tables controlled by different storage engines; such deadlocks are not detected automatically.

Answer 163

The innodb\_lock\_wait\_timeout option sets the balance between waiting for shared resources to become available, or giving up and handling the error, retrying, or doing alternative processing in your application. Rolls back any InnoDB transaction that waits more than a specified time to acquire a lock. Especially useful if deadlocks are caused by updates to multiple tables controlled by different storage engines; such deadlocks are not detected automatically.

Answer 164

The innodb\_lock\_wait\_timeout option sets the balance between waiting for shared resources to become available, or giving up and handling the error, retrying, or doing alternative processing in your application. Rolls back any InnoDB transaction that waits more than a specified time to acquire a lock. Especially useful if deadlocks are caused by updates to multiple tables controlled by different storage engines; such deadlocks are not detected automatically.

Answer 165

The innodb\_strict\_mode option controls whether InnoDB operates in strict mode, where conditions that are normally treated as warnings, cause errors instead (and the underlying statements fail). This mode is the default setting in MySQL 5.5.5 and higher.

Answer 166

The innodb\_strict\_mode option controls whether InnoDB operates in strict mode, where conditions that are normally treated as warnings, cause errors instead (and the underlying statements fail). This mode is the default setting in MySQL 5.5.5 and higher.

Answer 167

The innodb\_strict\_mode option controls whether InnoDB operates in strict mode, where conditions that are normally treated as warnings, cause errors instead (and the underlying statements fail). This mode is the default setting in MySQL 5.5.5 and higher.

Answer 168

One of the primary DML operations in SQL. The performance of inserts is a key factor in data warehouse systems that load millions of rows into tables, and OLTP systems where many concurrent connections might insert rows into the same table, in arbitrary order. If insert performance is important to you, you should learn about InnoDB features such as the insert buffer used in change buffering, and auto-increment columns.

Answer 169

One of the primary DML operations in SQL. The performance of inserts is a key factor in data warehouse systems that load millions of rows into tables, and OLTP systems where many concurrent connections might insert rows into the same table, in arbitrary order. If insert performance is important to you, you should learn about InnoDB features such as the insert buffer used in change buffering, and auto-increment columns.

Answer 170

One of the primary DML operations in SQL. The performance of inserts is a key factor in data warehouse systems that load millions of rows into tables, and OLTP systems where many concurrent connections might insert rows into the same table, in arbitrary order. If insert performance is important to you, you should learn about InnoDB features such as the insert buffer used in change buffering, and auto-increment columns.

Answer 171

Former name for the change buffer. Now that change buffering includes delete and update operations as well as inserts, "change buffer" is the preferred term.

Answer 172

Former name for the change buffer. Now that change buffering includes delete and update operations as well as inserts, "change buffer" is the preferred term.

Answer 173

Former name for the change buffer. Now that change buffering includes delete and update operations as well as inserts, "change buffer" is the preferred term.

Answer 174

The technique of storing secondary index changes due to INSERT operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. It is one of the types of change buffering; the others are delete buffering and purge buffering. Insert buffering is not used if the secondary index is unique, because the uniqueness of new values cannot be verified before the new entries are written out. Other kinds of change buffering do work for unique indexes.

Answer 175

The technique of storing secondary index changes due to INSERT operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. It is one of the types of change buffering; the others are delete buffering and purge buffering. Insert buffering is not used if the secondary index is unique, because the uniqueness of new values cannot be verified before the new entries are written out. Other kinds of change buffering do work for unique indexes.

Answer 176

The technique of storing secondary index changes due to INSERT operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. It is one of the types of change buffering; the others are delete buffering and purge buffering. Insert buffering is not used if the secondary index is unique, because the uniqueness of new values cannot be verified before the new entries are written out. Other kinds of change buffering do work for unique indexes.

Answer 177

A single mysqld daemon managing a data directory representing one or more databases with a set of tables. It is common in development, testing, and some replication scenarios to have multiple instances on the same server machine, each managing its own data directory and listening on its own port or socket. With one instance running a disk-bound workload, the server might still have extra CPU and memory capacity to run additional instances.

Answer 178

A single mysqld daemon managing a data directory representing one or more databases with a set of tables. It is common in development, testing, and some replication scenarios to have multiple instances on the same server machine, each managing its own data directory and listening on its own port or socket. With one instance running a disk-bound workload, the server might still have extra CPU and memory capacity to run additional instances.

Answer 179

A single mysqld daemon managing a data directory representing one or more databases with a set of tables. It is common in development, testing, and some replication scenarios to have multiple instances on the same server machine, each managing its own data directory and listening on its own port or socket. With one instance running a disk-bound workload, the server might still have extra CPU and memory capacity to run additional instances.

Answer 180

Modifications at the source code level to collect performance data for tuning and debugging. In MySQL, data collected by instrumentation is exposed through a SQL interface using the INFORMATION\_SCHEMA and PERFORMANCE\_SCHEMA databases.

Answer 181

Modifications at the source code level to collect performance data for tuning and debugging. In MySQL, data collected by instrumentation is exposed through a SQL interface using the INFORMATION\_SCHEMA and PERFORMANCE\_SCHEMA databases.

Answer 182

Modifications at the source code level to collect performance data for tuning and debugging. In MySQL, data collected by instrumentation is exposed through a SQL interface using the INFORMATION\_SCHEMA and PERFORMANCE\_SCHEMA databases.

Answer 183

A kind of lock that applies to the table level, used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible. For more details on this locking mechanism, seeSection 14.2.3, “InnoDB Lock Modes”.

Answer 184

A kind of lock that applies to the table level, used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible. For more details on this locking mechanism, seeSection 14.2.3, “InnoDB Lock Modes”.

Answer 185

A kind of lock that applies to the table level, used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible. For more details on this locking mechanism, seeSection 14.2.3, “InnoDB Lock Modes”.

Answer 186

A data structure optimized for document retrieval systems, used in the implementation of InnoDB full-text search. The InnoDB FULLTEXT index, implemented as an inverted index, records the position of each word within a document, rather than the location of a table row. A single column value (a document stored as a text string) is represented by many entries in the inverted index.

Answer 187

A data structure optimized for document retrieval systems, used in the implementation of InnoDB full-text search. The InnoDB FULLTEXT index, implemented as an inverted index, records the position of each word within a document, rather than the location of a table row. A single column value (a document stored as a text string) is represented by many entries in the inverted index.

Answer 188

A data structure optimized for document retrieval systems, used in the implementation of InnoDB full-text search. The InnoDB FULLTEXT index, implemented as an inverted index, records the position of each word within a document, rather than the location of a table row. A single column value (a document stored as a text string) is represented by many entries in the inverted index.

Answer 189

Acronym for I/O operations per second. A common measurement for busy systems, particularly OLTP applications. If this value is near the maximum that the storage devices can handle, the application can become disk-bound, limiting scalability.

Answer 190

Acronym for I/O operations per second. A common measurement for busy systems, particularly OLTP applications. If this value is near the maximum that the storage devices can handle, the application can become disk-bound, limiting scalability.

Answer 191

Acronym for I/O operations per second. A common measurement for busy systems, particularly OLTP applications. If this value is near the maximum that the storage devices can handle, the application can become disk-bound, limiting scalability.

Answer 192

One of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time. From highest amount of consistency and protection to the least, the isolation levels supported by InnoDB are: SERIALIZABLE, REPEATABLE READ, READ COMMITTED, and READ UNCOMMITTED. With InnoDB tables, many users can keep the default isolation level (REPEATABLE READ) for all operations. Expert users might choose the read committed level as they push the boundaries of scalability with OLTP processing, or during data warehousing operations where minor inconsistencies do not affect the aggregate results of large amounts of data. The levels on the edges (SERIALIZABLE and READ UNCOMMITTED) change the processing behavior to such an extent that they are rarely used.

Answer 193

One of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time. From highest amount of consistency and protection to the least, the isolation levels supported by InnoDB are: SERIALIZABLE, REPEATABLE READ, READ COMMITTED, and READ UNCOMMITTED. With InnoDB tables, many users can keep the default isolation level (REPEATABLE READ) for all operations. Expert users might choose the read committed level as they push the boundaries of scalability with OLTP processing, or during data warehousing operations where minor inconsistencies do not affect the aggregate results of large amounts of data. The levels on the edges (SERIALIZABLE and READ UNCOMMITTED) change the processing behavior to such an extent that they are rarely used.

Answer 194

One of the foundations of database processing. Isolation is the I in the acronym ACID; the isolation level is the setting that fine-tunes the balance between performance and reliability, consistency, and reproducibility of results when multiple transactions are making changes and performing queries at the same time. From highest amount of consistency and protection to the least, the isolation levels supported by InnoDB are: SERIALIZABLE, REPEATABLE READ, READ COMMITTED, and READ UNCOMMITTED. With InnoDB tables, many users can keep the default isolation level (REPEATABLE READ) for all operations. Expert users might choose the read committed level as they push the boundaries of scalability with OLTP processing, or during data warehousing operations where minor inconsistencies do not affect the aggregate results of large amounts of data. The levels on the edges (SERIALIZABLE and READ UNCOMMITTED) change the processing behavior to such an extent that they are rarely used.

Answer 195

A query that retrieves data from more than one table, by referencing columns in the tables that hold identical values. Ideally, these columns are part of an InnoDB foreign key relationship, which ensures referential integrity and that the join columns are indexed. Often used to save space and improve query performance by replacing repeated strings with numeric IDs, in a normalized data design.

Answer 196

A query that retrieves data from more than one table, by referencing columns in the tables that hold identical values. Ideally, these columns are part of an InnoDB foreign key relationship, which ensures referential integrity and that the join columns are indexed. Often used to save space and improve query performance by replacing repeated strings with numeric IDs, in a normalized data design.

Answer 197

A query that retrieves data from more than one table, by referencing columns in the tables that hold identical values. Ideally, these columns are part of an InnoDB foreign key relationship, which ensures referential integrity and that the join columns are indexed. Often used to save space and improve query performance by replacing repeated strings with numeric IDs, in a normalized data design.

Answer 198

An option to specify the size of data pages within an InnoDB table that uses compressed row format. The default is 8 kilobytes. Lower values risk hitting internal limits that depend on the combination of row size and compression percentage.

Answer 199

An option to specify the size of data pages within an InnoDB table that uses compressed row format. The default is 8 kilobytes. Lower values risk hitting internal limits that depend on the combination of row size and compression percentage.

Answer 200

An option to specify the size of data pages within an InnoDB table that uses compressed row format. The default is 8 kilobytes. Lower values risk hitting internal limits that depend on the combination of row size and compression percentage.

Answer 201

A lightweight structure used by InnoDB to implement a lock for its own internal memory structures, typically held for a brief time measured in milliseconds or microseconds. A general term that includes both mutexes (for exclusive access) and rw-locks (for shared access). Certain latches are the focus of InnoDB performance tuning, such as the data dictionary mutex. Statistics about latch use and contention are available through the Performance Schema interface.

Answer 202

A lightweight structure used by InnoDB to implement a lock for its own internal memory structures, typically held for a brief time measured in milliseconds or microseconds. A general term that includes both mutexes (for exclusive access) and rw-locks (for shared access). Certain latches are the focus of InnoDB performance tuning, such as the data dictionary mutex. Statistics about latch use and contention are available through the Performance Schema interface.

Answer 203

A lightweight structure used by InnoDB to implement a lock for its own internal memory structures, typically held for a brief time measured in milliseconds or microseconds. A general term that includes both mutexes (for exclusive access) and rw-locks (for shared access). Certain latches are the focus of InnoDB performance tuning, such as the data dictionary mutex. Statistics about latch use and contention are available through the Performance Schema interface.

Answer 204

The InnoDB buffer pool is represented as a list of memory pages. The list is reordered as new pages are accessed and enter the buffer pool, as pages within the buffer pool are accessed again and are considered newer, and as pages that are not accessed for a long time are evicted from the buffer pool. The buffer pool is actually divided into sublists, and the replacement policy is a variation of the familiar LRU technique.

Answer 205

The InnoDB buffer pool is represented as a list of memory pages. The list is reordered as new pages are accessed and enter the buffer pool, as pages within the buffer pool are accessed again and are considered newer, and as pages that are not accessed for a long time are evicted from the buffer pool. The buffer pool is actually divided into sublists, and the replacement policy is a variation of the familiar LRU technique.

Answer 206

The InnoDB buffer pool is represented as a list of memory pages. The list is reordered as new pages are accessed and enter the buffer pool, as pages within the buffer pool are accessed again and are considered newer, and as pages that are not accessed for a long time are evicted from the buffer pool. The buffer pool is actually divided into sublists, and the replacement policy is a variation of the familiar LRU technique.

Answer 207

The high-level notion of an object that controls access to a resource, such as a table, row, or internal data structure, as part of a locking strategy. For intensive performance tuning, you might delve into the actual structures that implement locks, such as mutexes and latches.

Answer 208

The high-level notion of an object that controls access to a resource, such as a table, row, or internal data structure, as part of a locking strategy. For intensive performance tuning, you might delve into the actual structures that implement locks, such as mutexes and latches.

Answer 209

The high-level notion of an object that controls access to a resource, such as a table, row, or internal data structure, as part of a locking strategy. For intensive performance tuning, you might delve into the actual structures that implement locks, such as mutexes and latches.

Answer 210

An operation used in some database systems that converts many row locks into a single table lock, saving memory space but reducing concurrent access to the table. InnoDB uses a space-efficient representation for row locks, so that lock escalation is not needed.

Answer 211

An operation used in some database systems that converts many row locks into a single table lock, saving memory space but reducing concurrent access to the table. InnoDB uses a space-efficient representation for row locks, so that lock escalation is not needed.

Answer 212

An operation used in some database systems that converts many row locks into a single table lock, saving memory space but reducing concurrent access to the table. InnoDB uses a space-efficient representation for row locks, so that lock escalation is not needed.

Answer 213

A shared (S) lock allows a transaction to read a row. Multiple transactions can acquire an S lock on that same row at the same time. An exclusive (X) lock allows a transaction to update or delete a row. No other transaction can acquire any kind of lock on that same row at the same time. Intention locks apply to the table level, and are used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible.

Answer 214

A shared (S) lock allows a transaction to read a row. Multiple transactions can acquire an S lock on that same row at the same time. An exclusive (X) lock allows a transaction to update or delete a row. No other transaction can acquire any kind of lock on that same row at the same time. Intention locks apply to the table level, and are used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible.

Answer 215

A shared (S) lock allows a transaction to read a row. Multiple transactions can acquire an S lock on that same row at the same time. An exclusive (X) lock allows a transaction to update or delete a row. No other transaction can acquire any kind of lock on that same row at the same time. Intention locks apply to the table level, and are used to indicate what kind of lock the transaction intends to acquire on rows in the table. Different transactions can acquire different kinds of intention locks on the same table, but the first transaction to acquire an intention exclusive (IX) lock on a table prevents other transactions from acquiring any S or X locks on the table. Conversely, the first transaction to acquire an intention shared (IS) lock on a table prevents other transactions from acquiring any X locks on the table. The two-phase process allows the lock requests to be resolved in order, without blocking locks and corresponding operations that are compatible.

Answer 216

The system of protecting a transaction from seeing or changing data that is being queried or changed by other transactions. The locking strategy must balance reliability and consistency of database operations (the principles of the ACID philosophy) against the performance needed for goodconcurrency. Fine-tuning the locking strategy often involves choosing an isolation level and ensuring all your database operations are safe and reliable for that isolation level.

Answer 217

The system of protecting a transaction from seeing or changing data that is being queried or changed by other transactions. The locking strategy must balance reliability and consistency of database operations (the principles of the ACID philosophy) against the performance needed for goodconcurrency. Fine-tuning the locking strategy often involves choosing an isolation level and ensuring all your database operations are safe and reliable for that isolation level.

Answer 218

The system of protecting a transaction from seeing or changing data that is being queried or changed by other transactions. The locking strategy must balance reliability and consistency of database operations (the principles of the ACID philosophy) against the performance needed for goodconcurrency. Fine-tuning the locking strategy often involves choosing an isolation level and ensuring all your database operations are safe and reliable for that isolation level.

Answer 219

A SELECT statement that also performs a locking operation on an InnoDB table. Either SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE. It has the potential to produce a deadlock, depending on the isolation level of the transaction. The opposite of a non-locking read. Not allowed for global tables in a read-only transaction.

Answer 220

A SELECT statement that also performs a locking operation on an InnoDB table. Either SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE. It has the potential to produce a deadlock, depending on the isolation level of the transaction. The opposite of a non-locking read. Not allowed for global tables in a read-only transaction.

Answer 221

A SELECT statement that also performs a locking operation on an InnoDB table. Either SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE. It has the potential to produce a deadlock, depending on the isolation level of the transaction. The opposite of a non-locking read. Not allowed for global tables in a read-only transaction.

Answer 222

In the InnoDB context, "log" or "log files" typically refers to the redo log represented by the ib\_logfile\* files. Another log area, which is physically part of the system tablespace, is the undo log. Other kinds of logs that are important in MySQL are the error log (for diagnosing startup and runtime problems), binary log (for working with replication and performing point-in-time restores), the general query log (for diagnosing application problems), and the slow query log (for diagnosing performance problems).

Answer 223

In the InnoDB context, "log" or "log files" typically refers to the redo log represented by the ib\_logfile\* files. Another log area, which is physically part of the system tablespace, is the undo log. Other kinds of logs that are important in MySQL are the error log (for diagnosing startup and runtime problems), binary log (for working with replication and performing point-in-time restores), the general query log (for diagnosing application problems), and the slow query log (for diagnosing performance problems).

Answer 224

In the InnoDB context, "log" or "log files" typically refers to the redo log represented by the ib\_logfile\* files. Another log area, which is physically part of the system tablespace, is the undo log. Other kinds of logs that are important in MySQL are the error log (for diagnosing startup and runtime problems), binary log (for working with replication and performing point-in-time restores), the general query log (for diagnosing application problems), and the slow query log (for diagnosing performance problems).

Answer 225

The memory area that holds data to be written to the log files that make up the redo log. It is controlled by theinnodb\_log\_buffer\_size configuration option.

Answer 226

The memory area that holds data to be written to the log files that make up the redo log. It is controlled by theinnodb\_log\_buffer\_size configuration option.

Answer 227

The memory area that holds data to be written to the log files that make up the redo log. It is controlled by theinnodb\_log\_buffer\_size configuration option.

Answer 228

One of the ib\_logfileN files that make up the redo log. Data is written to these files from the log buffer memory area.

Answer 229

One of the ib\_logfileN files that make up the redo log. Data is written to these files from the log buffer memory area.

Answer 230

One of the ib\_logfileN files that make up the redo log. Data is written to these files from the log buffer memory area.

Answer 231

The set of files that make up the redo log, typically named ib\_logfile0 and ib\_logfile1. (For that reason, sometimes referred to collectively as ib\_logfile.)

Answer 232

The set of files that make up the redo log, typically named ib\_logfile0 and ib\_logfile1. (For that reason, sometimes referred to collectively as ib\_logfile.)

Answer 233

The set of files that make up the redo log, typically named ib\_logfile0 and ib\_logfile1. (For that reason, sometimes referred to collectively as ib\_logfile.)

Answer 234

A type of operation that involves high-level, abstract aspects such as tables, queries, indexes, and other SQL concepts. Typically, logical aspects are important to make database administration and application development convenient and usable. Contrast with physical.

Answer 235

A type of operation that involves high-level, abstract aspects such as tables, queries, indexes, and other SQL concepts. Typically, logical aspects are important to make database administration and application development convenient and usable. Contrast with physical.

Answer 236

A type of operation that involves high-level, abstract aspects such as tables, queries, indexes, and other SQL concepts. Typically, logical aspects are important to make database administration and application development convenient and usable. Contrast with physical.

Answer 237

A backup that reproduces table structure and data, without copying the actual data files. For example, the mysqldump command produces a logical backup, because its output contains statements such as CREATE TABLE and INSERT that can re-create the data. Contrast withphysical backup. A logical backup offers flexibility (for example, you could edit table definitions or insert statements before restoring), but can take substantially longer to restore than a physical backup.

Answer 238

A backup that reproduces table structure and data, without copying the actual data files. For example, the mysqldump command produces a logical backup, because its output contains statements such as CREATE TABLE and INSERT that can re-create the data. Contrast withphysical backup. A logical backup offers flexibility (for example, you could edit table definitions or insert statements before restoring), but can take substantially longer to restore than a physical backup.

Answer 239

A backup that reproduces table structure and data, without copying the actual data files. For example, the mysqldump command produces a logical backup, because its output contains statements such as CREATE TABLE and INSERT that can re-create the data. Contrast withphysical backup. A logical backup offers flexibility (for example, you could edit table definitions or insert statements before restoring), but can take substantially longer to restore than a physical backup.

Answer 240

In MySQL 5.1, a prefix added to InnoDB configuration options when installing the InnoDB Plugin after server startup, so any new configuration options not recognized by the current level of MySQL do not cause a startup failure. MySQL processes configuration options that start with this prefix, but gives a warning rather than a failure if the part after the prefix is not a recognized option.

Answer 241

In MySQL 5.1, a prefix added to InnoDB configuration options when installing the InnoDB Plugin after server startup, so any new configuration options not recognized by the current level of MySQL do not cause a startup failure. MySQL processes configuration options that start with this prefix, but gives a warning rather than a failure if the part after the prefix is not a recognized option.

Answer 242

In MySQL 5.1, a prefix added to InnoDB configuration options when installing the InnoDB Plugin after server startup, so any new configuration options not recognized by the current level of MySQL do not cause a startup failure. MySQL processes configuration options that start with this prefix, but gives a warning rather than a failure if the part after the prefix is not a recognized option.

Answer 243

A value representing a lower limit, typically a threshold value at which some corrective action begins or becomes more aggressive. Contrast withhigh-water mark.

Answer 244

A value representing a lower limit, typically a threshold value at which some corrective action begins or becomes more aggressive. Contrast withhigh-water mark.

Answer 245

A value representing a lower limit, typically a threshold value at which some corrective action begins or becomes more aggressive. Contrast withhigh-water mark.

Answer 246

An acronym for "least recently used", a common method for managing storage areas. The items that have not been used recently are evictedwhen space is needed to cache newer items. InnoDB uses the LRU mechanism by default to manage the pages within the buffer pool, but makes exceptions in cases where a page might be read only a single time, such as during a full table scan. This variation of the LRU algorithm is called the midpoint insertion strategy. The ways in which the buffer pool management differs from the traditional LRU algorithm is fine-tuned by the options innodb\_old\_blocks\_pct, innodb\_old\_blocks\_time, and the new MySQL 5.6 optionsinnodb\_lru\_scan\_depth and innodb\_flush\_neighbors.

Answer 247

An acronym for "least recently used", a common method for managing storage areas. The items that have not been used recently are evictedwhen space is needed to cache newer items. InnoDB uses the LRU mechanism by default to manage the pages within the buffer pool, but makes exceptions in cases where a page might be read only a single time, such as during a full table scan. This variation of the LRU algorithm is called the midpoint insertion strategy. The ways in which the buffer pool management differs from the traditional LRU algorithm is fine-tuned by the options innodb\_old\_blocks\_pct, innodb\_old\_blocks\_time, and the new MySQL 5.6 optionsinnodb\_lru\_scan\_depth and innodb\_flush\_neighbors.

Answer 248

An acronym for "least recently used", a common method for managing storage areas. The items that have not been used recently are evictedwhen space is needed to cache newer items. InnoDB uses the LRU mechanism by default to manage the pages within the buffer pool, but makes exceptions in cases where a page might be read only a single time, such as during a full table scan. This variation of the LRU algorithm is called the midpoint insertion strategy. The ways in which the buffer pool management differs from the traditional LRU algorithm is fine-tuned by the options innodb\_old\_blocks\_pct, innodb\_old\_blocks\_time, and the new MySQL 5.6 optionsinnodb\_lru\_scan\_depth and innodb\_flush\_neighbors.

Answer 249

Acronym for "log sequence number". This arbitrary, ever-increasing value represents a point in time corresponding to operations recorded in the redo log. (This point in time is regardless of transaction boundaries; it can fall in the middle of one or more transactions.) It is used internally by InnoDB during crash recovery and for managing the buffer pool. Prior to MySQL 5.6.3, the LSN was a 4-byte unsigned integer. The LSN became an 8-byte unsigned integer in MySQL 5.6.3 when the redo log file size limit increased from 4GB to 512GB, as additional bytes were required to store extra size information. Applications built on MySQL 5.6.3 or later that use LSN values should use 64-bit rather than 32-bit variables to store and compare LSN values. In the MySQL Enterprise Backup product, you can specify an LSN to represent the point in time from which to take an incremental backup. The relevant LSN is displayed by the output of the mysqlbackup command. Once you have the LSN corresponding to the time of a full backup, you can specify that value to take a subsequent incremental backup, whose output contains another LSN for the next incremental backup.

Answer 250

Acronym for "log sequence number". This arbitrary, ever-increasing value represents a point in time corresponding to operations recorded in the redo log. (This point in time is regardless of transaction boundaries; it can fall in the middle of one or more transactions.) It is used internally by InnoDB during crash recovery and for managing the buffer pool. Prior to MySQL 5.6.3, the LSN was a 4-byte unsigned integer. The LSN became an 8-byte unsigned integer in MySQL 5.6.3 when the redo log file size limit increased from 4GB to 512GB, as additional bytes were required to store extra size information. Applications built on MySQL 5.6.3 or later that use LSN values should use 64-bit rather than 32-bit variables to store and compare LSN values. In the MySQL Enterprise Backup product, you can specify an LSN to represent the point in time from which to take an incremental backup. The relevant LSN is displayed by the output of the mysqlbackup command. Once you have the LSN corresponding to the time of a full backup, you can specify that value to take a subsequent incremental backup, whose output contains another LSN for the next incremental backup.

Answer 251

Acronym for "log sequence number". This arbitrary, ever-increasing value represents a point in time corresponding to operations recorded in the redo log. (This point in time is regardless of transaction boundaries; it can fall in the middle of one or more transactions.) It is used internally by InnoDB during crash recovery and for managing the buffer pool. Prior to MySQL 5.6.3, the LSN was a 4-byte unsigned integer. The LSN became an 8-byte unsigned integer in MySQL 5.6.3 when the redo log file size limit increased from 4GB to 512GB, as additional bytes were required to store extra size information. Applications built on MySQL 5.6.3 or later that use LSN values should use 64-bit rather than 32-bit variables to store and compare LSN values. In the MySQL Enterprise Backup product, you can specify an LSN to represent the point in time from which to take an incremental backup. The relevant LSN is displayed by the output of the mysqlbackup command. Once you have the LSN corresponding to the time of a full backup, you can specify that value to take a subsequent incremental backup, whose output contains another LSN for the next incremental backup.

Answer 252

Frequently shortened to "master". A database server machine in a replication scenario that processes the initial insert, update, and delete requests for data. These changes are propagated to, and repeated on, other servers known as slave servers.

Answer 253

Frequently shortened to "master". A database server machine in a replication scenario that processes the initial insert, update, and delete requests for data. These changes are propagated to, and repeated on, other servers known as slave servers.

Answer 254

Frequently shortened to "master". A database server machine in a replication scenario that processes the initial insert, update, and delete requests for data. These changes are propagated to, and repeated on, other servers known as slave servers.

Answer 255

An InnoDB thread that performs various tasks in the background. Most of these tasks are I/O related, such as writing changes from the insert buffer to the appropriate secondary indexes. To improve concurrency, sometimes actions are moved from the master thread to separate background threads. For example, in MySQL 5.6 and higher, dirty pages are flushed from the buffer pool by the page cleaner thread rather than the master thread.

Answer 256

An InnoDB thread that performs various tasks in the background. Most of these tasks are I/O related, such as writing changes from the insert buffer to the appropriate secondary indexes. To improve concurrency, sometimes actions are moved from the master thread to separate background threads. For example, in MySQL 5.6 and higher, dirty pages are flushed from the buffer pool by the page cleaner thread rather than the master thread.

Answer 257

An InnoDB thread that performs various tasks in the background. Most of these tasks are I/O related, such as writing changes from the insert buffer to the appropriate secondary indexes. To improve concurrency, sometimes actions are moved from the master thread to separate background threads. For example, in MySQL 5.6 and higher, dirty pages are flushed from the buffer pool by the page cleaner thread rather than the master thread.

Answer 258

A popular component of many MySQL and NoSQL software stacks, allowing fast reads and writes for single values and caching the results entirely in memory. Traditionally, applications required extra logic to write the same data to a MySQL database for permanent storage, or to read data from a MySQL database when it was not cached yet in memory. Now, applications can use the simple memcached protocol, supported by client libraries for many languages, to communicate directly with MySQL servers using InnoDB or MySQL Cluster tables. These NoSQL interfaces to MySQL tables allow applications to achieve higher read and write performance than by issuing SQL commands directly, and can simplify application logic and deployment configurations for systems that already incorporated memcached for in-memory caching. The memcached interface to InnoDB tables is available in MySQL 5.6 and higher; see Section 14.18, “InnoDB Integration with memcached” for details. The memcached interface to MySQL Cluster tables is available in MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html for details.

Answer 259

A popular component of many MySQL and NoSQL software stacks, allowing fast reads and writes for single values and caching the results entirely in memory. Traditionally, applications required extra logic to write the same data to a MySQL database for permanent storage, or to read data from a MySQL database when it was not cached yet in memory. Now, applications can use the simple memcached protocol, supported by client libraries for many languages, to communicate directly with MySQL servers using InnoDB or MySQL Cluster tables. These NoSQL interfaces to MySQL tables allow applications to achieve higher read and write performance than by issuing SQL commands directly, and can simplify application logic and deployment configurations for systems that already incorporated memcached for in-memory caching. The memcached interface to InnoDB tables is available in MySQL 5.6 and higher; see Section 14.18, “InnoDB Integration with memcached” for details. The memcached interface to MySQL Cluster tables is available in MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html for details.

Answer 260

A popular component of many MySQL and NoSQL software stacks, allowing fast reads and writes for single values and caching the results entirely in memory. Traditionally, applications required extra logic to write the same data to a MySQL database for permanent storage, or to read data from a MySQL database when it was not cached yet in memory. Now, applications can use the simple memcached protocol, supported by client libraries for many languages, to communicate directly with MySQL servers using InnoDB or MySQL Cluster tables. These NoSQL interfaces to MySQL tables allow applications to achieve higher read and write performance than by issuing SQL commands directly, and can simplify application logic and deployment configurations for systems that already incorporated memcached for in-memory caching. The memcached interface to InnoDB tables is available in MySQL 5.6 and higher; see Section 14.18, “InnoDB Integration with memcached” for details. The memcached interface to MySQL Cluster tables is available in MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.html for details.

Answer 261

To apply changes to data cached in memory, such as when a page is brought into the buffer pool, and any applicable changes recorded in thechange buffer are incorporated into the page in the buffer pool. The updated data is eventually written to the tablespace by the flushmechanism.

Answer 262

To apply changes to data cached in memory, such as when a page is brought into the buffer pool, and any applicable changes recorded in thechange buffer are incorporated into the page in the buffer pool. The updated data is eventually written to the tablespace by the flushmechanism.

Answer 263

To apply changes to data cached in memory, such as when a page is brought into the buffer pool, and any applicable changes recorded in thechange buffer are incorporated into the page in the buffer pool. The updated data is eventually written to the tablespace by the flushmechanism.

Answer 264

A type of lock that prevents DDL operations on a table that is being used at the same time by another transaction. For details, see Section 8.10.4, “Metadata Locking”. Enhancements to online operations, particularly in MySQL 5.6 and higher, are focused on reducing the amount of metadata locking. The objective is for DDL operations that do not change the table structure (such as CREATE INDEX and DROP INDEX for InnoDB tables) to proceed while the table is being queried, updated, and so on by other transactions.

Answer 265

A type of lock that prevents DDL operations on a table that is being used at the same time by another transaction. For details, see Section 8.10.4, “Metadata Locking”. Enhancements to online operations, particularly in MySQL 5.6 and higher, are focused on reducing the amount of metadata locking. The objective is for DDL operations that do not change the table structure (such as CREATE INDEX and DROP INDEX for InnoDB tables) to proceed while the table is being queried, updated, and so on by other transactions.

Answer 266

A type of lock that prevents DDL operations on a table that is being used at the same time by another transaction. For details, see Section 8.10.4, “Metadata Locking”. Enhancements to online operations, particularly in MySQL 5.6 and higher, are focused on reducing the amount of metadata locking. The objective is for DDL operations that do not change the table structure (such as CREATE INDEX and DROP INDEX for InnoDB tables) to proceed while the table is being queried, updated, and so on by other transactions.

Answer 267

A feature implemented by the innodb\_metrics table in the information\_schema, in MySQL 5.6 and higher. You can query counts and totals for low-level InnoDB operations, and use the results for performance tuning in combination with data from the performance\_schema.

Answer 268

A feature implemented by the innodb\_metrics table in the information\_schema, in MySQL 5.6 and higher. You can query counts and totals for low-level InnoDB operations, and use the results for performance tuning in combination with data from the performance\_schema.

Answer 269

A feature implemented by the innodb\_metrics table in the information\_schema, in MySQL 5.6 and higher. You can query counts and totals for low-level InnoDB operations, and use the results for performance tuning in combination with data from the performance\_schema.

Answer 270

The technique of initially bringing pages into the InnoDB buffer pool not at the "newest" end of the list, but instead somewhere in the middle. The exact location of this point can vary, based on the setting of the innodb\_old\_blocks\_pct option. The intent is that blocks that are only read once, such as during a full table scan, can be aged out of the buffer pool sooner than with a strict LRU algorithm.

Answer 271

The technique of initially bringing pages into the InnoDB buffer pool not at the "newest" end of the list, but instead somewhere in the middle. The exact location of this point can vary, based on the setting of the innodb\_old\_blocks\_pct option. The intent is that blocks that are only read once, such as during a full table scan, can be aged out of the buffer pool sooner than with a strict LRU algorithm.

Answer 272

The technique of initially bringing pages into the InnoDB buffer pool not at the "newest" end of the list, but instead somewhere in the middle. The exact location of this point can vary, based on the setting of the innodb\_old\_blocks\_pct option. The intent is that blocks that are only read once, such as during a full table scan, can be aged out of the buffer pool sooner than with a strict LRU algorithm.

Answer 273

An internal phase of InnoDB processing, when making changes at the physical level to internal data structures during DML operations. A mini-transaction (mtr) has no notion of rollback; multiple mini-transactions can occur within a single transaction. Mini-transactions write information to the redo log that is used during crash recovery. A mini-transaction can also happen outside the context of a regular transaction, for example during purge processing by background threads.

Answer 274

An internal phase of InnoDB processing, when making changes at the physical level to internal data structures during DML operations. A mini-transaction (mtr) has no notion of rollback; multiple mini-transactions can occur within a single transaction. Mini-transactions write information to the redo log that is used during crash recovery. A mini-transaction can also happen outside the context of a regular transaction, for example during purge processing by background threads.

Answer 275

An internal phase of InnoDB processing, when making changes at the physical level to internal data structures during DML operations. A mini-transaction (mtr) has no notion of rollback; multiple mini-transactions can occur within a single transaction. Mini-transactions write information to the redo log that is used during crash recovery. A mini-transaction can also happen outside the context of a regular transaction, for example during purge processing by background threads.

Answer 276

An INSERT statement where auto-increment values are specified for some but not all of the new rows. For example, a multi-value INSERT could specify a value for the auto-increment column in some cases and NULL in other cases. InnoDB generates auto-increment values for the rows where the column value was specified as NULL. Another example is an INSERT ... ON DUPLICATE KEY UPDATE statement, where auto-increment values might be generated but not used, for any duplicate rows that are processed as UPDATE rather than INSERT statements. Can cause consistency issues between master and slave servers in a replication configuration. Can require adjusting the value of the innodb\_autoinc\_lock\_mode configuration option.

Answer 277

An INSERT statement where auto-increment values are specified for some but not all of the new rows. For example, a multi-value INSERT could specify a value for the auto-increment column in some cases and NULL in other cases. InnoDB generates auto-increment values for the rows where the column value was specified as NULL. Another example is an INSERT ... ON DUPLICATE KEY UPDATE statement, where auto-increment values might be generated but not used, for any duplicate rows that are processed as UPDATE rather than INSERT statements. Can cause consistency issues between master and slave servers in a replication configuration. Can require adjusting the value of the innodb\_autoinc\_lock\_mode configuration option.

Answer 278

An INSERT statement where auto-increment values are specified for some but not all of the new rows. For example, a multi-value INSERT could specify a value for the auto-increment column in some cases and NULL in other cases. InnoDB generates auto-increment values for the rows where the column value was specified as NULL. Another example is an INSERT ... ON DUPLICATE KEY UPDATE statement, where auto-increment values might be generated but not used, for any duplicate rows that are processed as UPDATE rather than INSERT statements. Can cause consistency issues between master and slave servers in a replication configuration. Can require adjusting the value of the innodb\_autoinc\_lock\_mode configuration option.

Answer 279

A type of processor that can take advantage of multi-threaded programs, such as the MySQL server.

Answer 280

A type of processor that can take advantage of multi-threaded programs, such as the MySQL server.

Answer 281

A type of processor that can take advantage of multi-threaded programs, such as the MySQL server.

Answer 282

Informal abbreviation for "mutex variable". (Mutex itself is short for "mutual exclusion".) The low-level object that InnoDB uses to represent and enforce exclusive-access locks to internal in-memory data structures. Once the lock is acquired, any other process, thread, and so on is prevented from acquiring the same lock. Contrast with rw-locks, which allow shared access. Mutexes and rw-locks are known collectively aslatches.

Answer 283

Informal abbreviation for "mutex variable". (Mutex itself is short for "mutual exclusion".) The low-level object that InnoDB uses to represent and enforce exclusive-access locks to internal in-memory data structures. Once the lock is acquired, any other process, thread, and so on is prevented from acquiring the same lock. Contrast with rw-locks, which allow shared access. Mutexes and rw-locks are known collectively aslatches.

Answer 284

Informal abbreviation for "mutex variable". (Mutex itself is short for "mutual exclusion".) The low-level object that InnoDB uses to represent and enforce exclusive-access locks to internal in-memory data structures. Once the lock is acquired, any other process, thread, and so on is prevented from acquiring the same lock. Contrast with rw-locks, which allow shared access. Mutexes and rw-locks are known collectively aslatches.

Answer 285

Acronym for "multiversion concurrency control". This technique lets InnoDB transactions with certain isolation levels to perform consistent read operations; that is, to query rows that are being updated by other transactions, and see the values from before those updates occurred. This is a powerful technique to increase concurrency, by allowing queries to proceed without waiting due to locks held by the other transactions. This technique is not universal in the database world. Some other database products, and some other MySQL storage engines, do not support it.

Answer 286

Acronym for "multiversion concurrency control". This technique lets InnoDB transactions with certain isolation levels to perform consistent read operations; that is, to query rows that are being updated by other transactions, and see the values from before those updates occurred. This is a powerful technique to increase concurrency, by allowing queries to proceed without waiting due to locks held by the other transactions. This technique is not universal in the database world. Some other database products, and some other MySQL storage engines, do not support it.

Answer 287

Acronym for "multiversion concurrency control". This technique lets InnoDB transactions with certain isolation levels to perform consistent read operations; that is, to query rows that are being updated by other transactions, and see the values from before those updates occurred. This is a powerful technique to increase concurrency, by allowing queries to proceed without waiting due to locks held by the other transactions. This technique is not universal in the database world. Some other database products, and some other MySQL storage engines, do not support it.

Answer 288

The name, on UNIX or Linux systems, of the MySQL option file.

Answer 289

The name, on UNIX or Linux systems, of the MySQL option file.

Answer 290

The name, on UNIX or Linux systems, of the MySQL option file.

Answer 291

The name, on Windows systems, of the MySQL option file.

Answer 292

The name, on Windows systems, of the MySQL option file.

Answer 293

The name, on Windows systems, of the MySQL option file.

Answer 294

The mysql program is the command-line interpreter for the MySQL database. It processes SQL statements, and also MySQL-specific commands such as SHOW TABLES, by passing requests to the mysqld daemon.

Answer 295

The mysql program is the command-line interpreter for the MySQL database. It processes SQL statements, and also MySQL-specific commands such as SHOW TABLES, by passing requests to the mysqld daemon.

Answer 296

The mysql program is the command-line interpreter for the MySQL database. It processes SQL statements, and also MySQL-specific commands such as SHOW TABLES, by passing requests to the mysqld daemon.

Answer 297

A licensed product that performs hot backups of MySQL databases. It offers the most efficiency and flexibility when backing up InnoDB tables, but can also back up MyISAM and other kinds of tables.

Answer 298

A licensed product that performs hot backups of MySQL databases. It offers the most efficiency and flexibility when backing up InnoDB tables, but can also back up MyISAM and other kinds of tables.

Answer 299

A licensed product that performs hot backups of MySQL databases. It offers the most efficiency and flexibility when backing up InnoDB tables, but can also back up MyISAM and other kinds of tables.

Answer 300

A command-line tool of the MySQL Enterprise Backup product. It performs a hot backup operation for InnoDB tables, and a warm backup for MyISAM and other kinds of tables. See Section 25.2, “MySQL Enterprise Backup” for more information about this command.

Answer 301

A command-line tool of the MySQL Enterprise Backup product. It performs a hot backup operation for InnoDB tables, and a warm backup for MyISAM and other kinds of tables. See Section 25.2, “MySQL Enterprise Backup” for more information about this command.

Answer 302

A command-line tool of the MySQL Enterprise Backup product. It performs a hot backup operation for InnoDB tables, and a warm backup for MyISAM and other kinds of tables. See Section 25.2, “MySQL Enterprise Backup” for more information about this command.

Answer 303

The mysqld program is the database engine for the MySQL database. It runs as a UNIX daemon or Windows service, constantly waiting for requests and performing maintenance work in the background.

Answer 304

The mysqld program is the database engine for the MySQL database. It runs as a UNIX daemon or Windows service, constantly waiting for requests and performing maintenance work in the background.

Answer 305

The mysqld program is the database engine for the MySQL database. It runs as a UNIX daemon or Windows service, constantly waiting for requests and performing maintenance work in the background.

Answer 306

A command that performs a logical backup of some combination of databases, tables, and table data. The results are SQL statements that reproduce the original schema objects, data, or both. For substantial amounts of data, a physical backup solution such as MySQL Enterprise Backup is faster, particularly for the restore operation.

Answer 307

A command that performs a logical backup of some combination of databases, tables, and table data. The results are SQL statements that reproduce the original schema objects, data, or both. For substantial amounts of data, a physical backup solution such as MySQL Enterprise Backup is faster, particularly for the restore operation.

Answer 308

A command that performs a logical backup of some combination of databases, tables, and table data. The results are SQL statements that reproduce the original schema objects, data, or both. For substantial amounts of data, a physical backup solution such as MySQL Enterprise Backup is faster, particularly for the restore operation.

Answer 309

A indexed column, typically a primary key, where the values have some real-world significance. Usually advised against because: If the value should ever change, there is potentially a lot of index maintenance to re-sort the clustered index and update the copies of the primary key value that are repeated in each secondary index. Even seemingly stable values can change in unpredictable ways that are difficult to represent correctly in the database. For example, one country can change into two or several, making the original country code obsolete. Or, rules about unique values might have exceptions. For example, even if taxpayer IDs are intended to be unique to a single person, a database might have to handle records that violate that rule, such as in cases of identity theft. Taxpayer IDs and other sensitive ID numbers also make poor primary keys, because they may need to be secured, encrypted, and otherwise treated differently than other columns. Thus, it is typically better to use arbitrary numeric values to form a synthetic key, for example using an auto-increment column.

Answer 310

A indexed column, typically a primary key, where the values have some real-world significance. Usually advised against because: If the value should ever change, there is potentially a lot of index maintenance to re-sort the clustered index and update the copies of the primary key value that are repeated in each secondary index. Even seemingly stable values can change in unpredictable ways that are difficult to represent correctly in the database. For example, one country can change into two or several, making the original country code obsolete. Or, rules about unique values might have exceptions. For example, even if taxpayer IDs are intended to be unique to a single person, a database might have to handle records that violate that rule, such as in cases of identity theft. Taxpayer IDs and other sensitive ID numbers also make poor primary keys, because they may need to be secured, encrypted, and otherwise treated differently than other columns. Thus, it is typically better to use arbitrary numeric values to form a synthetic key, for example using an auto-increment column.

Answer 311

A indexed column, typically a primary key, where the values have some real-world significance. Usually advised against because: If the value should ever change, there is potentially a lot of index maintenance to re-sort the clustered index and update the copies of the primary key value that are repeated in each secondary index. Even seemingly stable values can change in unpredictable ways that are difficult to represent correctly in the database. For example, one country can change into two or several, making the original country code obsolete. Or, rules about unique values might have exceptions. For example, even if taxpayer IDs are intended to be unique to a single person, a database might have to handle records that violate that rule, such as in cases of identity theft. Taxpayer IDs and other sensitive ID numbers also make poor primary keys, because they may need to be secured, encrypted, and otherwise treated differently than other columns. Thus, it is typically better to use arbitrary numeric values to form a synthetic key, for example using an auto-increment column.

Answer 312

Any page in the same extent as a particular page. When a page is selected to be flushed, any neighbor pages that are dirty are typically flushed as well, as an I/O optimization for traditional hard disks. In MySQL 5.6 and up, this behavior can be controlled by the configuration variableinnodb\_flush\_neighbors; you might turn that setting off for SSD drives, which do not have the same overhead for writing smaller batches of data at random locations.

Answer 313

Any page in the same extent as a particular page. When a page is selected to be flushed, any neighbor pages that are dirty are typically flushed as well, as an I/O optimization for traditional hard disks. In MySQL 5.6 and up, this behavior can be controlled by the configuration variableinnodb\_flush\_neighbors; you might turn that setting off for SSD drives, which do not have the same overhead for writing smaller batches of data at random locations.

Answer 314

Any page in the same extent as a particular page. When a page is selected to be flushed, any neighbor pages that are dirty are typically flushed as well, as an I/O optimization for traditional hard disks. In MySQL 5.6 and up, this behavior can be controlled by the configuration variableinnodb\_flush\_neighbors; you might turn that setting off for SSD drives, which do not have the same overhead for writing smaller batches of data at random locations.

Answer 315

A combination of a record lock on the index record and a gap lock on the gap before the index record.

Answer 316

A combination of a record lock on the index record and a gap lock on the gap before the index record.

Answer 317

A combination of a record lock on the index record and a gap lock on the gap before the index record.

Answer 318

An industry term that means the same as asynchronous I/O.

Answer 319

An industry term that means the same as asynchronous I/O.

Answer 320

An industry term that means the same as asynchronous I/O.

Answer 321

A query that does not use the SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE clauses. The only kind of query allowed for global tables in a read-only transaction. The opposite of a locking read.

Answer 322

A query that does not use the SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE clauses. The only kind of query allowed for global tables in a read-only transaction. The opposite of a locking read.

Answer 323

A query that does not use the SELECT ... FOR UPDATE or SELECT ... LOCK IN SHARE MODE clauses. The only kind of query allowed for global tables in a read-only transaction. The opposite of a locking read.

Answer 324

The situation when a query retrieves data, and a later query within the same transaction retrieves what should be the same data, but the queries return different results (changed by another transaction committing in the meantime). This kind of operation goes against the ACID principle of database design. Within a transaction, data should be consistent, with predictable and stable relationships. Among different isolation levels, non-repeatable reads are prevented by the serializable read and repeatable read levels, and allowed by the consistent read, and read uncommitted levels.

Answer 325

The situation when a query retrieves data, and a later query within the same transaction retrieves what should be the same data, but the queries return different results (changed by another transaction committing in the meantime). This kind of operation goes against the ACID principle of database design. Within a transaction, data should be consistent, with predictable and stable relationships. Among different isolation levels, non-repeatable reads are prevented by the serializable read and repeatable read levels, and allowed by the consistent read, and read uncommitted levels.

Answer 326

The situation when a query retrieves data, and a later query within the same transaction retrieves what should be the same data, but the queries return different results (changed by another transaction committing in the meantime). This kind of operation goes against the ACID principle of database design. Within a transaction, data should be consistent, with predictable and stable relationships. Among different isolation levels, non-repeatable reads are prevented by the serializable read and repeatable read levels, and allowed by the consistent read, and read uncommitted levels.

Answer 327

A database design strategy where data is split into multiple tables, and duplicate values condensed into single rows represented by an ID, to avoid storing, querying, and updating redundant or lengthy values. It is typically used in OLTP applications. For example, an address might be given a unique ID, so that a census database could represent the relationship lives at this address by associating that ID with each member of a family, rather than storing multiple copies of a complex value such as 123 Main Street, Anytown, USA. For another example, although a simple address book application might store each phone number in the same table as a person's name and address, a phone company database might give each phone number a special ID, and store the numbers and IDs in a separate table. This normalized representation could simplify large-scale updates when area codes split apart. Normalization is not always recommended. Data that is primarily queried, and only updated by deleting entirely and reloading, is often kept in fewer, larger tables with redundant copies of duplicate values. This data representation is referred to as denormalized, and is frequently found in data warehousing applications.

Answer 328

A database design strategy where data is split into multiple tables, and duplicate values condensed into single rows represented by an ID, to avoid storing, querying, and updating redundant or lengthy values. It is typically used in OLTP applications. For example, an address might be given a unique ID, so that a census database could represent the relationship lives at this address by associating that ID with each member of a family, rather than storing multiple copies of a complex value such as 123 Main Street, Anytown, USA. For another example, although a simple address book application might store each phone number in the same table as a person's name and address, a phone company database might give each phone number a special ID, and store the numbers and IDs in a separate table. This normalized representation could simplify large-scale updates when area codes split apart. Normalization is not always recommended. Data that is primarily queried, and only updated by deleting entirely and reloading, is often kept in fewer, larger tables with redundant copies of duplicate values. This data representation is referred to as denormalized, and is frequently found in data warehousing applications.

Answer 329

A database design strategy where data is split into multiple tables, and duplicate values condensed into single rows represented by an ID, to avoid storing, querying, and updating redundant or lengthy values. It is typically used in OLTP applications. For example, an address might be given a unique ID, so that a census database could represent the relationship lives at this address by associating that ID with each member of a family, rather than storing multiple copies of a complex value such as 123 Main Street, Anytown, USA. For another example, although a simple address book application might store each phone number in the same table as a person's name and address, a phone company database might give each phone number a special ID, and store the numbers and IDs in a separate table. This normalized representation could simplify large-scale updates when area codes split apart. Normalization is not always recommended. Data that is primarily queried, and only updated by deleting entirely and reloading, is often kept in fewer, larger tables with redundant copies of duplicate values. This data representation is referred to as denormalized, and is frequently found in data warehousing applications.

Answer 330

A broad term for a set of data access technologies that do not use the SQL language as their primary mechanism for reading and writing data. Some NoSQL technologies act as key-value stores, only accepting single-value reads and writes; some relax the restrictions of the ACIDmethodology; still others do not require a pre-planned schema. MySQL users can combine NoSQL-style processing for speed and simplicity with SQL operations for flexibility and convenience, by using the memcached API to directly access some kinds of MySQL tables. The memcachedinterface to InnoDB tables is available in MySQL 5.6 and higher; see Section 14.18, “InnoDB Integration with memcached” for details. Thememcached interface to MySQL Cluster tables is available in MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.htmlfor details.

Answer 331

A broad term for a set of data access technologies that do not use the SQL language as their primary mechanism for reading and writing data. Some NoSQL technologies act as key-value stores, only accepting single-value reads and writes; some relax the restrictions of the ACIDmethodology; still others do not require a pre-planned schema. MySQL users can combine NoSQL-style processing for speed and simplicity with SQL operations for flexibility and convenience, by using the memcached API to directly access some kinds of MySQL tables. The memcachedinterface to InnoDB tables is available in MySQL 5.6 and higher; see Section 14.18, “InnoDB Integration with memcached” for details. Thememcached interface to MySQL Cluster tables is available in MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.htmlfor details.

Answer 332

A broad term for a set of data access technologies that do not use the SQL language as their primary mechanism for reading and writing data. Some NoSQL technologies act as key-value stores, only accepting single-value reads and writes; some relax the restrictions of the ACIDmethodology; still others do not require a pre-planned schema. MySQL users can combine NoSQL-style processing for speed and simplicity with SQL operations for flexibility and convenience, by using the memcached API to directly access some kinds of MySQL tables. The memcachedinterface to InnoDB tables is available in MySQL 5.6 and higher; see Section 14.18, “InnoDB Integration with memcached” for details. Thememcached interface to MySQL Cluster tables is available in MySQL Cluster 7.2; see http://dev.mysql.com/doc/ndbapi/en/ndbmemcache.htmlfor details.

Answer 333

A type of constraint that specifies that a column cannot contain any NULL values. It helps to preserve referential integrity, as the database server can identify data with erroneous missing values. It also helps in the arithmetic involved in query optimization, allowing the optimizer to predict the number of entries in an index on that column.

Answer 334

A type of constraint that specifies that a column cannot contain any NULL values. It helps to preserve referential integrity, as the database server can identify data with erroneous missing values. It also helps in the arithmetic involved in query optimization, allowing the optimizer to predict the number of entries in an index on that column.

Answer 335

A type of constraint that specifies that a column cannot contain any NULL values. It helps to preserve referential integrity, as the database server can identify data with erroneous missing values. It also helps in the arithmetic involved in query optimization, allowing the optimizer to predict the number of entries in an index on that column.

Answer 336

A special value in SQL, indicating the absence of data. Any arithmetic operation or equality test involving a NULL value, in turn produces a NULL result. (Thus it is similar to the IEEE floating-point concept of NaN, "not a number".) Any aggregate calculation such as AVG() ignores rows with NULL values, when determining how many rows to divide by. The only test that works with NULL values uses the SQL idioms IS NULL or IS NOT NULL. NULL values play a part in index operations, because for performance a database must minimize the overhead of keeping track of missing data values. Typically, NULL values are not stored in an index, because a query that tests an indexed column using a standard comparison operator could never match a row with a NULL value for that column. For the same reason, unique indexes do not prevent NULL values; those values simply are not represented in the index. Declaring a NOT NULL constraint on a column provides reassurance that there are no rows left out of the index, allowing for better query optimization (accurate counting of rows and estimation of whether to use the index). Because the primary key must be able to uniquely identify every row in the table, a single-column primary key cannot contain any NULL values, and a multi-column primary key cannot contain any rows with NULL values in all columns. Although the Oracle database allows a NULL value to be concatenated with a string, InnoDB treats the result of such an operation as NULL.

Answer 337

A special value in SQL, indicating the absence of data. Any arithmetic operation or equality test involving a NULL value, in turn produces a NULL result. (Thus it is similar to the IEEE floating-point concept of NaN, "not a number".) Any aggregate calculation such as AVG() ignores rows with NULL values, when determining how many rows to divide by. The only test that works with NULL values uses the SQL idioms IS NULL or IS NOT NULL. NULL values play a part in index operations, because for performance a database must minimize the overhead of keeping track of missing data values. Typically, NULL values are not stored in an index, because a query that tests an indexed column using a standard comparison operator could never match a row with a NULL value for that column. For the same reason, unique indexes do not prevent NULL values; those values simply are not represented in the index. Declaring a NOT NULL constraint on a column provides reassurance that there are no rows left out of the index, allowing for better query optimization (accurate counting of rows and estimation of whether to use the index). Because the primary key must be able to uniquely identify every row in the table, a single-column primary key cannot contain any NULL values, and a multi-column primary key cannot contain any rows with NULL values in all columns. Although the Oracle database allows a NULL value to be concatenated with a string, InnoDB treats the result of such an operation as NULL.

Answer 338

A special value in SQL, indicating the absence of data. Any arithmetic operation or equality test involving a NULL value, in turn produces a NULL result. (Thus it is similar to the IEEE floating-point concept of NaN, "not a number".) Any aggregate calculation such as AVG() ignores rows with NULL values, when determining how many rows to divide by. The only test that works with NULL values uses the SQL idioms IS NULL or IS NOT NULL. NULL values play a part in index operations, because for performance a database must minimize the overhead of keeping track of missing data values. Typically, NULL values are not stored in an index, because a query that tests an indexed column using a standard comparison operator could never match a row with a NULL value for that column. For the same reason, unique indexes do not prevent NULL values; those values simply are not represented in the index. Declaring a NOT NULL constraint on a column provides reassurance that there are no rows left out of the index, allowing for better query optimization (accurate counting of rows and estimation of whether to use the index). Because the primary key must be able to uniquely identify every row in the table, a single-column primary key cannot contain any NULL values, and a multi-column primary key cannot contain any rows with NULL values in all columns. Although the Oracle database allows a NULL value to be concatenated with a string, InnoDB treats the result of such an operation as NULL.

Answer 339

A column containing variable-length data (such as BLOB and VARCHAR) that is too long to fit on a B-tree page. The data is stored in overflow pages. The DYNAMIC row format in the InnoDB Barracuda file format is more efficient for such storage than the older COMPACT row format.

Answer 340

A column containing variable-length data (such as BLOB and VARCHAR) that is too long to fit on a B-tree page. The data is stored in overflow pages. The DYNAMIC row format in the InnoDB Barracuda file format is more efficient for such storage than the older COMPACT row format.

Answer 341

A column containing variable-length data (such as BLOB and VARCHAR) that is too long to fit on a B-tree page. The data is stored in overflow pages. The DYNAMIC row format in the InnoDB Barracuda file format is more efficient for such storage than the older COMPACT row format.

Answer 342

Acronym for "Online Transaction Processing". A database system, or a database application, that runs a workload with many transactions, with frequent writes as well as reads, typically affecting small amounts of data at a time. For example, an airline reservation system or an application that processes bank deposits. The data might be organized in normalized form for a balance between DML (insert/update/delete) efficiency and query efficiency. Contrast with data warehouse. With its row-level locking and transactional capability, InnoDB is the ideal storage engine for MySQL tables used in OLTP applications.

Answer 343

Acronym for "Online Transaction Processing". A database system, or a database application, that runs a workload with many transactions, with frequent writes as well as reads, typically affecting small amounts of data at a time. For example, an airline reservation system or an application that processes bank deposits. The data might be organized in normalized form for a balance between DML (insert/update/delete) efficiency and query efficiency. Contrast with data warehouse. With its row-level locking and transactional capability, InnoDB is the ideal storage engine for MySQL tables used in OLTP applications.

Answer 344

Acronym for "Online Transaction Processing". A database system, or a database application, that runs a workload with many transactions, with frequent writes as well as reads, typically affecting small amounts of data at a time. For example, an airline reservation system or an application that processes bank deposits. The data might be organized in normalized form for a balance between DML (insert/update/delete) efficiency and query efficiency. Contrast with data warehouse. With its row-level locking and transactional capability, InnoDB is the ideal storage engine for MySQL tables used in OLTP applications.

Answer 345

A type of operation that involves no downtime, blocking, or restricted operation for the database. Typically applied to DDL. Operations that shorten the periods of restricted operation, such as fast index creation, have evolved into a wider set of online DDL operations in MySQL 5.6. In the context of backups, a hot backup is an online operation and a warm backup is partially an online operation.

Answer 346

A type of operation that involves no downtime, blocking, or restricted operation for the database. Typically applied to DDL. Operations that shorten the periods of restricted operation, such as fast index creation, have evolved into a wider set of online DDL operations in MySQL 5.6. In the context of backups, a hot backup is an online operation and a warm backup is partially an online operation.

Answer 347

A type of operation that involves no downtime, blocking, or restricted operation for the database. Typically applied to DDL. Operations that shorten the periods of restricted operation, such as fast index creation, have evolved into a wider set of online DDL operations in MySQL 5.6. In the context of backups, a hot backup is an online operation and a warm backup is partially an online operation.

Answer 348

A feature that improves the performance, concurrency, and availability of InnoDB tables during DDL (primarily ALTER TABLE) operations. See Section 14.11, “InnoDB and Online DDL” for details. The details vary according to the type of operation. In some cases, the table can be modified concurrently while the ALTER TABLE is in progress. The operation might be able to be performed without doing a table copy, or using a specially optimized type of table copy. Space usage is controlled by the innodb\_online\_alter\_log\_max\_size configuration option. This feature is an enhancement of the Fast Index Creation feature in MySQL 5.5 and the InnoDB Plugin for MySQL 5.1.

Answer 349

A feature that improves the performance, concurrency, and availability of InnoDB tables during DDL (primarily ALTER TABLE) operations. See Section 14.11, “InnoDB and Online DDL” for details. The details vary according to the type of operation. In some cases, the table can be modified concurrently while the ALTER TABLE is in progress. The operation might be able to be performed without doing a table copy, or using a specially optimized type of table copy. Space usage is controlled by the innodb\_online\_alter\_log\_max\_size configuration option. This feature is an enhancement of the Fast Index Creation feature in MySQL 5.5 and the InnoDB Plugin for MySQL 5.1.

Answer 350

A feature that improves the performance, concurrency, and availability of InnoDB tables during DDL (primarily ALTER TABLE) operations. See Section 14.11, “InnoDB and Online DDL” for details. The details vary according to the type of operation. In some cases, the table can be modified concurrently while the ALTER TABLE is in progress. The operation might be able to be performed without doing a table copy, or using a specially optimized type of table copy. Space usage is controlled by the innodb\_online\_alter\_log\_max\_size configuration option. This feature is an enhancement of the Fast Index Creation feature in MySQL 5.5 and the InnoDB Plugin for MySQL 5.1.

Answer 351

A methodology that guides low-level implementation decisions for a relational database system. The requirements of performance and concurrency in a relational database mean that operations must be started or dispatched quickly. The requirements of consistency and referential integrity mean that any operation could fail: a transaction might be rolled back, a DML operation could violate a constraint, a request for a lock could cause a deadlock, a network error could cause a timeout. An optimistic strategy is one that assumes most requests or attempts will succeed, so that relatively little work is done to prepare for the failure case. When this assumption is true, the database does little unnecessary work; when requests do fail, extra work must be done to clean up and undo changes. InnoDB uses optimistic strategies for operations such as locking and commits. For example, data changed by a transaction can be written to the data files before the commit occurs, making the commit itself very fast, but requiring more work to undo the changes if the transaction is rolled back. The opposite of an optimistic strategy is a pessimistic one, where a system is optimized to deal with operations that are unreliable and frequently unsuccessful. This methodology is rare in a database system, because so much care goes into choosing reliable hardware, networks, and algorithms.

Answer 352

A methodology that guides low-level implementation decisions for a relational database system. The requirements of performance and concurrency in a relational database mean that operations must be started or dispatched quickly. The requirements of consistency and referential integrity mean that any operation could fail: a transaction might be rolled back, a DML operation could violate a constraint, a request for a lock could cause a deadlock, a network error could cause a timeout. An optimistic strategy is one that assumes most requests or attempts will succeed, so that relatively little work is done to prepare for the failure case. When this assumption is true, the database does little unnecessary work; when requests do fail, extra work must be done to clean up and undo changes. InnoDB uses optimistic strategies for operations such as locking and commits. For example, data changed by a transaction can be written to the data files before the commit occurs, making the commit itself very fast, but requiring more work to undo the changes if the transaction is rolled back. The opposite of an optimistic strategy is a pessimistic one, where a system is optimized to deal with operations that are unreliable and frequently unsuccessful. This methodology is rare in a database system, because so much care goes into choosing reliable hardware, networks, and algorithms.

Answer 353

A methodology that guides low-level implementation decisions for a relational database system. The requirements of performance and concurrency in a relational database mean that operations must be started or dispatched quickly. The requirements of consistency and referential integrity mean that any operation could fail: a transaction might be rolled back, a DML operation could violate a constraint, a request for a lock could cause a deadlock, a network error could cause a timeout. An optimistic strategy is one that assumes most requests or attempts will succeed, so that relatively little work is done to prepare for the failure case. When this assumption is true, the database does little unnecessary work; when requests do fail, extra work must be done to clean up and undo changes. InnoDB uses optimistic strategies for operations such as locking and commits. For example, data changed by a transaction can be written to the data files before the commit occurs, making the commit itself very fast, but requiring more work to undo the changes if the transaction is rolled back. The opposite of an optimistic strategy is a pessimistic one, where a system is optimized to deal with operations that are unreliable and frequently unsuccessful. This methodology is rare in a database system, because so much care goes into choosing reliable hardware, networks, and algorithms.

Answer 354

The MySQL component that determines the best indexes and join order to use for a query, based on characteristics and data distribution of the relevant tables.

Answer 355

The MySQL component that determines the best indexes and join order to use for a query, based on characteristics and data distribution of the relevant tables.

Answer 356

The MySQL component that determines the best indexes and join order to use for a query, based on characteristics and data distribution of the relevant tables.

Answer 357

A configuration parameter for MySQL, either stored in the option file or passed on the command line. For the options that apply to InnoDB tables, each option name starts with the prefix innodb\_.

Answer 358

A configuration parameter for MySQL, either stored in the option file or passed on the command line. For the options that apply to InnoDB tables, each option name starts with the prefix innodb\_.

Answer 359

A configuration parameter for MySQL, either stored in the option file or passed on the command line. For the options that apply to InnoDB tables, each option name starts with the prefix innodb\_.

Answer 360

The file that holds the configuration options for the MySQL instance. Traditionally, on Linux and UNIX this file is named my.cnf, and on Windows it is named my.ini.

Answer 361

The file that holds the configuration options for the MySQL instance. Traditionally, on Linux and UNIX this file is named my.cnf, and on Windows it is named my.ini.

Answer 362

The file that holds the configuration options for the MySQL instance. Traditionally, on Linux and UNIX this file is named my.cnf, and on Windows it is named my.ini.

Answer 363

Separately allocated disk pages that hold variable-length columns (such as BLOB and VARCHAR) that are too long to fit on a B-tree page. The associated columns are known as off-page columns.

Answer 364

Separately allocated disk pages that hold variable-length columns (such as BLOB and VARCHAR) that are too long to fit on a B-tree page. The associated columns are known as off-page columns.

Answer 365

Separately allocated disk pages that hold variable-length columns (such as BLOB and VARCHAR) that are too long to fit on a B-tree page. The associated columns are known as off-page columns.

Answer 366

A unit representing how much data InnoDB transfers at any one time between disk (the data files) and memory (the buffer pool). A page can contain one or more rows, depending on how much data is in each row. If a row does not fit entirely into a single page, InnoDB sets up additional pointer-style data structures so that the information about the row can be stored in one page. One way to fit more data in each page is to use compressed row format. For tables that use BLOBs or large text fields, compact row format allows those large columns to be stored separately from the rest of the row, reducing I/O overhead and memory usage for queries that do not reference those columns. When InnoDB reads or writes sets of pages as a batch to increase I/O throughput, it reads or writes an extent at a time. All the InnoDB disk data structures within a MySQL instance share the same page size.

Answer 367

A unit representing how much data InnoDB transfers at any one time between disk (the data files) and memory (the buffer pool). A page can contain one or more rows, depending on how much data is in each row. If a row does not fit entirely into a single page, InnoDB sets up additional pointer-style data structures so that the information about the row can be stored in one page. One way to fit more data in each page is to use compressed row format. For tables that use BLOBs or large text fields, compact row format allows those large columns to be stored separately from the rest of the row, reducing I/O overhead and memory usage for queries that do not reference those columns. When InnoDB reads or writes sets of pages as a batch to increase I/O throughput, it reads or writes an extent at a time. All the InnoDB disk data structures within a MySQL instance share the same page size.

Answer 368

A unit representing how much data InnoDB transfers at any one time between disk (the data files) and memory (the buffer pool). A page can contain one or more rows, depending on how much data is in each row. If a row does not fit entirely into a single page, InnoDB sets up additional pointer-style data structures so that the information about the row can be stored in one page. One way to fit more data in each page is to use compressed row format. For tables that use BLOBs or large text fields, compact row format allows those large columns to be stored separately from the rest of the row, reducing I/O overhead and memory usage for queries that do not reference those columns. When InnoDB reads or writes sets of pages as a batch to increase I/O throughput, it reads or writes an extent at a time. All the InnoDB disk data structures within a MySQL instance share the same page size.

Answer 369

An InnoDB background thread that flushes dirty pages from the buffer pool. Prior to MySQL 5.6, this activity was performed by the master thread

Answer 370

An InnoDB background thread that flushes dirty pages from the buffer pool. Prior to MySQL 5.6, this activity was performed by the master thread

Answer 371

An InnoDB background thread that flushes dirty pages from the buffer pool. Prior to MySQL 5.6, this activity was performed by the master thread

Answer 372

For releases up to and including MySQL 5.5, the size of each InnoDB page is fixed at 16 kilobytes. This value represents a balance: large enough to hold the data for most rows, yet small enough to minimize the performance overhead of transferring unneeded data to memory. Other values are not tested or supported. Starting in MySQL 5.6, the page size for an InnoDB instance can be either 4KB, 8KB, or 16KB, controlled by the innodb\_page\_size configuration option. You set the size when creating the MySQL instance, and it remains constant afterwards. The same page size applies to all InnoDB tablespaces, both the system tablespace and any separate tablespaces created in file-per-table mode. Smaller page sizes can help performance with storage devices that use small block sizes, particularly for SSD devices in disk-bound workloads, such as for OLTP applications. As individual rows are updated, less data is copied into memory, written to disk, reorganized, locked, and so on.

Answer 373

For releases up to and including MySQL 5.5, the size of each InnoDB page is fixed at 16 kilobytes. This value represents a balance: large enough to hold the data for most rows, yet small enough to minimize the performance overhead of transferring unneeded data to memory. Other values are not tested or supported. Starting in MySQL 5.6, the page size for an InnoDB instance can be either 4KB, 8KB, or 16KB, controlled by the innodb\_page\_size configuration option. You set the size when creating the MySQL instance, and it remains constant afterwards. The same page size applies to all InnoDB tablespaces, both the system tablespace and any separate tablespaces created in file-per-table mode. Smaller page sizes can help performance with storage devices that use small block sizes, particularly for SSD devices in disk-bound workloads, such as for OLTP applications. As individual rows are updated, less data is copied into memory, written to disk, reorganized, locked, and so on.

Answer 374

For releases up to and including MySQL 5.5, the size of each InnoDB page is fixed at 16 kilobytes. This value represents a balance: large enough to hold the data for most rows, yet small enough to minimize the performance overhead of transferring unneeded data to memory. Other values are not tested or supported. Starting in MySQL 5.6, the page size for an InnoDB instance can be either 4KB, 8KB, or 16KB, controlled by the innodb\_page\_size configuration option. You set the size when creating the MySQL instance, and it remains constant afterwards. The same page size applies to all InnoDB tablespaces, both the system tablespace and any separate tablespaces created in file-per-table mode. Smaller page sizes can help performance with storage devices that use small block sizes, particularly for SSD devices in disk-bound workloads, such as for OLTP applications. As individual rows are updated, less data is copied into memory, written to disk, reorganized, locked, and so on.

Answer 375

The table in a foreign key relationship that holds the initial column values pointed to from the child table. The consequences of deleting, or updating rows in the parent table depend on the ON UPDATE and ON DELETE clauses in the foreign key definition. Rows with corresponding values in the child table could be automatically deleted or updated in turn, or those columns could be set to NULL, or the operation could be prevented.

Answer 376

The table in a foreign key relationship that holds the initial column values pointed to from the child table. The consequences of deleting, or updating rows in the parent table depend on the ON UPDATE and ON DELETE clauses in the foreign key definition. Rows with corresponding values in the child table could be automatically deleted or updated in turn, or those columns could be set to NULL, or the operation could be prevented.

Answer 377

The table in a foreign key relationship that holds the initial column values pointed to from the child table. The consequences of deleting, or updating rows in the parent table depend on the ON UPDATE and ON DELETE clauses in the foreign key definition. Rows with corresponding values in the child table could be automatically deleted or updated in turn, or those columns could be set to NULL, or the operation could be prevented.

Answer 378

A backup that contains some of the tables in a MySQL database, or some of the databases in a MySQL instance. Contrast with full backup.

Answer 379

A backup that contains some of the tables in a MySQL database, or some of the databases in a MySQL instance. Contrast with full backup.

Answer 380

A backup that contains some of the tables in a MySQL database, or some of the databases in a MySQL instance. Contrast with full backup.

Answer 381

An index that represents only part of a column value, typically the first N characters (the prefix) of a long VARCHAR value.

Answer 382

An index that represents only part of a column value, typically the first N characters (the prefix) of a long VARCHAR value.

Answer 383

An index that represents only part of a column value, typically the first N characters (the prefix) of a long VARCHAR value.

Answer 384

The performance\_schema schema, in MySQL 5.5 and up, presents a set of tables that you can query to get detailed information about the performance characteristics of many internal parts of the MySQL server.

Answer 385

The performance\_schema schema, in MySQL 5.5 and up, presents a set of tables that you can query to get detailed information about the performance characteristics of many internal parts of the MySQL server.

Answer 386

The performance\_schema schema, in MySQL 5.5 and up, presents a set of tables that you can query to get detailed information about the performance characteristics of many internal parts of the MySQL server.

Answer 387

A feature in MySQL 5.6 that stores index statistics for InnoDB tables on disk, providing better plan stability for queries. For more information, see Section 14.13.17.1, “Configuring Persistent Optimizer Statistics Parameters”.

Answer 388

A feature in MySQL 5.6 that stores index statistics for InnoDB tables on disk, providing better plan stability for queries. For more information, see Section 14.13.17.1, “Configuring Persistent Optimizer Statistics Parameters”.

Answer 389

A feature in MySQL 5.6 that stores index statistics for InnoDB tables on disk, providing better plan stability for queries. For more information, see Section 14.13.17.1, “Configuring Persistent Optimizer Statistics Parameters”.

Answer 390

A methodology that sacrifices performance or concurrency in favor of safety. It is appropriate if a high proportion of requests or attempts might fail, or if the consequences of a failed request are severe. InnoDB uses what is known as a pessimistic locking strategy, to minimize the chance of deadlocks. At the application level, you might avoid deadlocks by using a pessimistic strategy of acquiring all locks needed by a transaction at the very beginning. Many built-in database mechanisms use the opposite optimistic methodology.

Answer 391

A methodology that sacrifices performance or concurrency in favor of safety. It is appropriate if a high proportion of requests or attempts might fail, or if the consequences of a failed request are severe. InnoDB uses what is known as a pessimistic locking strategy, to minimize the chance of deadlocks. At the application level, you might avoid deadlocks by using a pessimistic strategy of acquiring all locks needed by a transaction at the very beginning. Many built-in database mechanisms use the opposite optimistic methodology.

Answer 392

A methodology that sacrifices performance or concurrency in favor of safety. It is appropriate if a high proportion of requests or attempts might fail, or if the consequences of a failed request are severe. InnoDB uses what is known as a pessimistic locking strategy, to minimize the chance of deadlocks. At the application level, you might avoid deadlocks by using a pessimistic strategy of acquiring all locks needed by a transaction at the very beginning. Many built-in database mechanisms use the opposite optimistic methodology.

Answer 393

A row that appears in the result set of a query, but not in the result set of an earlier query. For example, if a query is run twice within a transaction, and in the meantime, another transaction commits after inserting a new row or updating a row so that it matches the WHERE clause of the query. This occurrence is known as a phantom read. It is harder to guard against than a non-repeatable read, because locking all the rows from the first query result set does not prevent the changes that cause the phantom to appear. Among different isolation levels, phantom reads are prevented by the serializable read level, and allowed by the repeatable read, consistent read, and read uncommitted levels.

Answer 394

A row that appears in the result set of a query, but not in the result set of an earlier query. For example, if a query is run twice within a transaction, and in the meantime, another transaction commits after inserting a new row or updating a row so that it matches the WHERE clause of the query. This occurrence is known as a phantom read. It is harder to guard against than a non-repeatable read, because locking all the rows from the first query result set does not prevent the changes that cause the phantom to appear. Among different isolation levels, phantom reads are prevented by the serializable read level, and allowed by the repeatable read, consistent read, and read uncommitted levels.

Answer 395

A row that appears in the result set of a query, but not in the result set of an earlier query. For example, if a query is run twice within a transaction, and in the meantime, another transaction commits after inserting a new row or updating a row so that it matches the WHERE clause of the query. This occurrence is known as a phantom read. It is harder to guard against than a non-repeatable read, because locking all the rows from the first query result set does not prevent the changes that cause the phantom to appear. Among different isolation levels, phantom reads are prevented by the serializable read level, and allowed by the repeatable read, consistent read, and read uncommitted levels.

Answer 396

A type of operation that involves hardware-related aspects such as disk blocks, memory pages, files, bits, disk reads, and so on. Typically, physical aspects are important during expert-level performance tuning and problem diagnosis. Contrast with logical.

Answer 397

A type of operation that involves hardware-related aspects such as disk blocks, memory pages, files, bits, disk reads, and so on. Typically, physical aspects are important during expert-level performance tuning and problem diagnosis. Contrast with logical.

Answer 398

A type of operation that involves hardware-related aspects such as disk blocks, memory pages, files, bits, disk reads, and so on. Typically, physical aspects are important during expert-level performance tuning and problem diagnosis. Contrast with logical.

Answer 399

A backup that copies the actual data files. For example, the mysqlbackup command of the MySQL Enterprise Backup product produces a physical backup, because its output contains data files that can be used directly by the mysqld server, resulting in a faster restore operation. Contrast with logical backup.

Answer 400

A backup that copies the actual data files. For example, the mysqlbackup command of the MySQL Enterprise Backup product produces a physical backup, because its output contains data files that can be used directly by the mysqld server, resulting in a faster restore operation. Contrast with logical backup.

Answer 401

A backup that copies the actual data files. For example, the mysqlbackup command of the MySQL Enterprise Backup product produces a physical backup, because its output contains data files that can be used directly by the mysqld server, resulting in a faster restore operation. Contrast with logical backup.

Answer 402

Acronym for point-in-time recovery.

Answer 403

Acronym for point-in-time recovery.

Answer 404

Acronym for point-in-time recovery.

Answer 405

A property of a query execution plan, where the optimizer makes the same choices each time for a given query, so that performance is consistent and predictable.

Answer 406

A property of a query execution plan, where the optimizer makes the same choices each time for a given query, so that performance is consistent and predictable.

Answer 407

A property of a query execution plan, where the optimizer makes the same choices each time for a given query, so that performance is consistent and predictable.

Answer 408

In MySQL 5.1 and earlier, a separately installable form of the InnoDB storage engine that includes features and performance enhancements not included in the built-in InnoDB for those releases. For MySQL 5.5 and higher, the MySQL distribution includes the very latest InnoDB features and performance enhancements, known as InnoDB 1.1, and there is no longer a separate InnoDB Plugin. This distinction is important mainly in MySQL 5.1, where a feature or bug fix might apply to the InnoDB Plugin but not the built-in InnoDB, or vice versa.

Answer 409

In MySQL 5.1 and earlier, a separately installable form of the InnoDB storage engine that includes features and performance enhancements not included in the built-in InnoDB for those releases. For MySQL 5.5 and higher, the MySQL distribution includes the very latest InnoDB features and performance enhancements, known as InnoDB 1.1, and there is no longer a separate InnoDB Plugin. This distinction is important mainly in MySQL 5.1, where a feature or bug fix might apply to the InnoDB Plugin but not the built-in InnoDB, or vice versa.

Answer 410

In MySQL 5.1 and earlier, a separately installable form of the InnoDB storage engine that includes features and performance enhancements not included in the built-in InnoDB for those releases. For MySQL 5.5 and higher, the MySQL distribution includes the very latest InnoDB features and performance enhancements, known as InnoDB 1.1, and there is no longer a separate InnoDB Plugin. This distinction is important mainly in MySQL 5.1, where a feature or bug fix might apply to the InnoDB Plugin but not the built-in InnoDB, or vice versa.

Answer 411

The process of restoring a backup to recreate the state of the database at a specific date and time. Commonly abbreviated PITR. Because it is unlikely that the specified time corresponds exactly to the time of a backup, this technique usually requires a combination of a physical backupand a logical backup. For example, with the MySQL Enterprise Backup product, you restore the last backup that you took before the specified point in time, then replay changes from the binary log between the time of the backup and the PITR time.

Answer 412

The process of restoring a backup to recreate the state of the database at a specific date and time. Commonly abbreviated PITR. Because it is unlikely that the specified time corresponds exactly to the time of a backup, this technique usually requires a combination of a physical backupand a logical backup. For example, with the MySQL Enterprise Backup product, you restore the last backup that you took before the specified point in time, then replay changes from the binary log between the time of the backup and the PITR time.

Answer 413

The process of restoring a backup to recreate the state of the database at a specific date and time. Commonly abbreviated PITR. Because it is unlikely that the specified time corresponds exactly to the time of a backup, this technique usually requires a combination of a physical backupand a logical backup. For example, with the MySQL Enterprise Backup product, you restore the last backup that you took before the specified point in time, then replay changes from the binary log between the time of the backup and the PITR time.

Answer 414

See index prefix.

Answer 415

See index prefix.

Answer 416

See index prefix.

Answer 417

A set of backup files, produced by the MySQL Enterprise Backup product, after all the stages of applying binary logs and incremental backups are finished. The resulting files are ready to be restored. Prior to the apply steps, the files are known as a raw backup.

Answer 418

A set of backup files, produced by the MySQL Enterprise Backup product, after all the stages of applying binary logs and incremental backups are finished. The resulting files are ready to be restored. Prior to the apply steps, the files are known as a raw backup.

Answer 419

A set of backup files, produced by the MySQL Enterprise Backup product, after all the stages of applying binary logs and incremental backups are finished. The resulting files are ready to be restored. Prior to the apply steps, the files are known as a raw backup.

Answer 420

A set of columns -- and by implication, the index based on this set of columns -- that can uniquely identify every row in a table. As such, it must be a unique index that does not contain any NULL values. InnoDB requires that every table has such an index (also called the clustered index or cluster index), and organizes the table storage based on the column values of the primary key. When choosing primary key values, consider using arbitrary values (a synthetic key) rather than relying on values derived from some other source (a natural key).

Answer 421

A set of columns -- and by implication, the index based on this set of columns -- that can uniquely identify every row in a table. As such, it must be a unique index that does not contain any NULL values. InnoDB requires that every table has such an index (also called the clustered index or cluster index), and organizes the table storage based on the column values of the primary key. When choosing primary key values, consider using arbitrary values (a synthetic key) rather than relying on values derived from some other source (a natural key).

Answer 422

A set of columns -- and by implication, the index based on this set of columns -- that can uniquely identify every row in a table. As such, it must be a unique index that does not contain any NULL values. InnoDB requires that every table has such an index (also called the clustered index or cluster index), and organizes the table storage based on the column values of the primary key. When choosing primary key values, consider using arbitrary values (a synthetic key) rather than relying on values derived from some other source (a natural key).

Answer 423

An instance of an executing program. The operating system switches between multiple running processes, allowing for a certain degree ofconcurrency. On most operating systems, processes can contain multiple threads of execution that share resources. Context-switching between threads is faster than the equivalent switching between processes.

Answer 424

An instance of an executing program. The operating system switches between multiple running processes, allowing for a certain degree ofconcurrency. On most operating systems, processes can contain multiple threads of execution that share resources. Context-switching between threads is faster than the equivalent switching between processes.

Answer 425

An instance of an executing program. The operating system switches between multiple running processes, allowing for a certain degree ofconcurrency. On most operating systems, processes can contain multiple threads of execution that share resources. Context-switching between threads is faster than the equivalent switching between processes.

Answer 426

An artificial record in an index, used for locking key values or ranges that do not currently exist.

Answer 427

An artificial record in an index, used for locking key values or ranges that do not currently exist.

Answer 428

An artificial record in an index, used for locking key values or ranges that do not currently exist.

Answer 429

The POSIX threads standard, which defines an API for threading and locking operations on UNIX and Linux systems. On UNIX and Linux systems, InnoDB uses this implementation for mutexes.

Answer 430

The POSIX threads standard, which defines an API for threading and locking operations on UNIX and Linux systems. On UNIX and Linux systems, InnoDB uses this implementation for mutexes.

Answer 431

The POSIX threads standard, which defines an API for threading and locking operations on UNIX and Linux systems. On UNIX and Linux systems, InnoDB uses this implementation for mutexes.

Answer 432

A type of garbage collection performed by a separate thread, running on a periodic schedule. The purge includes these actions: removing obsolete values from indexes; physically removing rows that were marked for deletion by previous DELETE statements.

Answer 433

A type of garbage collection performed by a separate thread, running on a periodic schedule. The purge includes these actions: removing obsolete values from indexes; physically removing rows that were marked for deletion by previous DELETE statements.

Answer 434

A type of garbage collection performed by a separate thread, running on a periodic schedule. The purge includes these actions: removing obsolete values from indexes; physically removing rows that were marked for deletion by previous DELETE statements.

Answer 435

The technique of storing index changes due to DELETE operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. (Because delete operations are a two-step process, this operation buffers the write that normally purges an index record that was previously marked for deletion.) It is one of the types of change buffering; the others are insert buffering anddelete buffering.

Answer 436

The technique of storing index changes due to DELETE operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. (Because delete operations are a two-step process, this operation buffers the write that normally purges an index record that was previously marked for deletion.) It is one of the types of change buffering; the others are insert buffering anddelete buffering.

Answer 437

The technique of storing index changes due to DELETE operations in the insert buffer rather than writing them immediately, so that the physical writes can be performed to minimize random I/O. (Because delete operations are a two-step process, this operation buffers the write that normally purges an index record that was previously marked for deletion.) It is one of the types of change buffering; the others are insert buffering anddelete buffering.

Answer 438

Another name for the InnoDB history list. Related to the innodb\_max\_purge\_lag configuration option.

Answer 439

Another name for the InnoDB history list. Related to the innodb\_max\_purge\_lag configuration option.

Answer 440

Another name for the InnoDB history list. Related to the innodb\_max\_purge\_lag configuration option.

Answer 441

A thread within the InnoDB process that is dedicated to performing the periodic purge operation. In MySQL 5.6 and higher, multiple purge threads are enabled by the innodb\_purge\_threads configuration option.

Answer 442

A thread within the InnoDB process that is dedicated to performing the periodic purge operation. In MySQL 5.6 and higher, multiple purge threads are enabled by the innodb\_purge\_threads configuration option.

Answer 443

A thread within the InnoDB process that is dedicated to performing the periodic purge operation. In MySQL 5.6 and higher, multiple purge threads are enabled by the innodb\_purge\_threads configuration option.

Answer 444

In SQL, an operation that reads information from one or more tables. Depending on the organization of data and the parameters of the query, the lookup might be optimized by consulting an index. If multiple tables are involved, the query is known as a join. For historical reasons, sometimes discussions of internal processing for statements use "query" in a broader sense, including other types of MySQL statements such as DDL and DML statements.

Answer 445

In SQL, an operation that reads information from one or more tables. Depending on the organization of data and the parameters of the query, the lookup might be optimized by consulting an index. If multiple tables are involved, the query is known as a join. For historical reasons, sometimes discussions of internal processing for statements use "query" in a broader sense, including other types of MySQL statements such as DDL and DML statements.

Answer 446

In SQL, an operation that reads information from one or more tables. Depending on the organization of data and the parameters of the query, the lookup might be optimized by consulting an index. If multiple tables are involved, the query is known as a join. For historical reasons, sometimes discussions of internal processing for statements use "query" in a broader sense, including other types of MySQL statements such as DDL and DML statements.

Answer 447

The set of decisions made by the optimizer about how to perform a query most efficiently, including which index or indexes to use, and the order in which to join tables. Plan stability involves the same choices being made consistently for a given query.

Answer 448

The set of decisions made by the optimizer about how to perform a query most efficiently, including which index or indexes to use, and the order in which to join tables. Plan stability involves the same choices being made consistently for a given query.

Answer 449

The set of decisions made by the optimizer about how to perform a query most efficiently, including which index or indexes to use, and the order in which to join tables. Plan stability involves the same choices being made consistently for a given query.

Answer 450

To reduce the amount of database activity, often in preparation for an operation such as an ALTER TABLE, a backup, or a shutdown. Might or might not involve doing as much flushing as possible, so that InnoDB does not continue doing background I/O. In MySQL 5.6 and higher, the syntax FLUSH TABLES ... FOR EXPORT writes some data to disk for InnoDB tables that make it simpler to back up those tables by copying the data files.

Answer 451

To reduce the amount of database activity, often in preparation for an operation such as an ALTER TABLE, a backup, or a shutdown. Might or might not involve doing as much flushing as possible, so that InnoDB does not continue doing background I/O. In MySQL 5.6 and higher, the syntax FLUSH TABLES ... FOR EXPORT writes some data to disk for InnoDB tables that make it simpler to back up those tables by copying the data files.

Answer 452

To reduce the amount of database activity, often in preparation for an operation such as an ALTER TABLE, a backup, or a shutdown. Might or might not involve doing as much flushing as possible, so that InnoDB does not continue doing background I/O. In MySQL 5.6 and higher, the syntax FLUSH TABLES ... FOR EXPORT writes some data to disk for InnoDB tables that make it simpler to back up those tables by copying the data files.

Answer 453

Acronym for "Redundant Array of Inexpensive Drives". Spreading I/O operations across multiple drives enables greater concurrency at the hardware level, and improves the efficiency of low-level write operations that otherwise would be performed in sequence.

Answer 454

Acronym for "Redundant Array of Inexpensive Drives". Spreading I/O operations across multiple drives enables greater concurrency at the hardware level, and improves the efficiency of low-level write operations that otherwise would be performed in sequence.

Answer 455

Acronym for "Redundant Array of Inexpensive Drives". Spreading I/O operations across multiple drives enables greater concurrency at the hardware level, and improves the efficiency of low-level write operations that otherwise would be performed in sequence.

Answer 456

A technique for quickly estimating the number of different values in a column (the column's cardinality). InnoDB samples pages at random from the index and uses that data to estimate the number of different values. This operation occurs when each table is first opened. Originally, the number of sampled pages was fixed at 8; now, it is determined by the setting of the innodb\_stats\_sample\_pages parameter. The way the random pages are picked depends on the setting of the innodb\_use\_legacy\_cardinality\_algorithm parameter. The default setting (OFF) has better randomness than in older releases.

Answer 457

A technique for quickly estimating the number of different values in a column (the column's cardinality). InnoDB samples pages at random from the index and uses that data to estimate the number of different values. This operation occurs when each table is first opened. Originally, the number of sampled pages was fixed at 8; now, it is determined by the setting of the innodb\_stats\_sample\_pages parameter. The way the random pages are picked depends on the setting of the innodb\_use\_legacy\_cardinality\_algorithm parameter. The default setting (OFF) has better randomness than in older releases.

Answer 458

A technique for quickly estimating the number of different values in a column (the column's cardinality). InnoDB samples pages at random from the index and uses that data to estimate the number of different values. This operation occurs when each table is first opened. Originally, the number of sampled pages was fixed at 8; now, it is determined by the setting of the innodb\_stats\_sample\_pages parameter. The way the random pages are picked depends on the setting of the innodb\_use\_legacy\_cardinality\_algorithm parameter. The default setting (OFF) has better randomness than in older releases.

Answer 459

The initial set of backup files produced by the MySQL Enterprise Backup product, before the changes reflected in the binary log and anyincremental backups are applied. At this stage, the files are not ready to restore. After these changes are applied, the files are known as aprepared backup.

Answer 460

The initial set of backup files produced by the MySQL Enterprise Backup product, before the changes reflected in the binary log and anyincremental backups are applied. At this stage, the files are not ready to restore. After these changes are applied, the files are known as aprepared backup.

Answer 461

The initial set of backup files produced by the MySQL Enterprise Backup product, before the changes reflected in the binary log and anyincremental backups are applied. At this stage, the files are not ready to restore. After these changes are applied, the files are known as aprepared backup.

Answer 462

An isolation level that uses a locking strategy that relaxes some of the protection between transactions, in the interest of performance. Transactions cannot see uncommitted data from other transactions, but they can see data that is committed by another transaction after the current transaction started. Thus, a transaction never sees any bad data, but the data that it does see may depend to some extent on the timing of other transactions. When a transaction with this isolation level performs UPDATE ... WHERE or DELETE ... WHERE operations, other transactions might have to wait. The transaction can perform SELECT ... FOR UPDATE, and LOCK IN SHARE MODE operations without making other transactions wait.

Answer 463

An isolation level that uses a locking strategy that relaxes some of the protection between transactions, in the interest of performance. Transactions cannot see uncommitted data from other transactions, but they can see data that is committed by another transaction after the current transaction started. Thus, a transaction never sees any bad data, but the data that it does see may depend to some extent on the timing of other transactions. When a transaction with this isolation level performs UPDATE ... WHERE or DELETE ... WHERE operations, other transactions might have to wait. The transaction can perform SELECT ... FOR UPDATE, and LOCK IN SHARE MODE operations without making other transactions wait.

Answer 464

An isolation level that uses a locking strategy that relaxes some of the protection between transactions, in the interest of performance. Transactions cannot see uncommitted data from other transactions, but they can see data that is committed by another transaction after the current transaction started. Thus, a transaction never sees any bad data, but the data that it does see may depend to some extent on the timing of other transactions. When a transaction with this isolation level performs UPDATE ... WHERE or DELETE ... WHERE operations, other transactions might have to wait. The transaction can perform SELECT ... FOR UPDATE, and LOCK IN SHARE MODE operations without making other transactions wait.

Answer 465

The isolation level that provides the least amount of protection between transactions. Queries employ a locking strategy that allows them to proceed in situations where they would normally wait for another transaction. However, this extra performance comes at the cost of less reliable results, including data that has been changed by other transactions and not committed yet (known as dirty read). Use this isolation level only with great caution, and be aware that the results might not be consistent or reproducible, depending on what other transactions are doing at the same time. Typically, transactions with this isolation level do only queries, not insert, update, or delete operations.

Answer 466

The isolation level that provides the least amount of protection between transactions. Queries employ a locking strategy that allows them to proceed in situations where they would normally wait for another transaction. However, this extra performance comes at the cost of less reliable results, including data that has been changed by other transactions and not committed yet (known as dirty read). Use this isolation level only with great caution, and be aware that the results might not be consistent or reproducible, depending on what other transactions are doing at the same time. Typically, transactions with this isolation level do only queries, not insert, update, or delete operations.

Answer 467

The isolation level that provides the least amount of protection between transactions. Queries employ a locking strategy that allows them to proceed in situations where they would normally wait for another transaction. However, this extra performance comes at the cost of less reliable results, including data that has been changed by other transactions and not committed yet (known as dirty read). Use this isolation level only with great caution, and be aware that the results might not be consistent or reproducible, depending on what other transactions are doing at the same time. Typically, transactions with this isolation level do only queries, not insert, update, or delete operations.

Answer 468

An internal snapshot used by the MVCC mechanism of InnoDB. Certain transactions, depending on their isolation level, see the data values as they were at the time the transaction (or in some cases, the statement) started. Isolation levels that use a read view are REPEATABLE READ,READ COMMITTED, and READ UNCOMMITTED.

Answer 469

An internal snapshot used by the MVCC mechanism of InnoDB. Certain transactions, depending on their isolation level, see the data values as they were at the time the transaction (or in some cases, the statement) started. Isolation levels that use a read view are REPEATABLE READ,READ COMMITTED, and READ UNCOMMITTED.

Answer 470

An internal snapshot used by the MVCC mechanism of InnoDB. Certain transactions, depending on their isolation level, see the data values as they were at the time the transaction (or in some cases, the statement) started. Isolation levels that use a read view are REPEATABLE READ,READ COMMITTED, and READ UNCOMMITTED.

Answer 471

A type of I/O request that prefetches a group of pages (an entire extent) into the buffer pool asynchronously, in anticipation that these pages will be needed soon. The linear read-ahead technique prefetches all the pages of one extent based on access patterns for pages in the preceding extent, and is part of all MySQL versions starting with the InnoDB Plugin for MySQL 5.1. The random read-ahead technique prefetches all the pages for an extent once a certain number of pages from the same extent are in the buffer pool. Random read-ahead is not part of MySQL 5.5, but is re-introduced in MySQL 5.6 under the control of the innodb\_random\_read\_ahead configuration option.

Answer 472

A type of I/O request that prefetches a group of pages (an entire extent) into the buffer pool asynchronously, in anticipation that these pages will be needed soon. The linear read-ahead technique prefetches all the pages of one extent based on access patterns for pages in the preceding extent, and is part of all MySQL versions starting with the InnoDB Plugin for MySQL 5.1. The random read-ahead technique prefetches all the pages for an extent once a certain number of pages from the same extent are in the buffer pool. Random read-ahead is not part of MySQL 5.5, but is re-introduced in MySQL 5.6 under the control of the innodb\_random\_read\_ahead configuration option.

Answer 473

A type of I/O request that prefetches a group of pages (an entire extent) into the buffer pool asynchronously, in anticipation that these pages will be needed soon. The linear read-ahead technique prefetches all the pages of one extent based on access patterns for pages in the preceding extent, and is part of all MySQL versions starting with the InnoDB Plugin for MySQL 5.1. The random read-ahead technique prefetches all the pages for an extent once a certain number of pages from the same extent are in the buffer pool. Random read-ahead is not part of MySQL 5.5, but is re-introduced in MySQL 5.6 under the control of the innodb\_random\_read\_ahead configuration option.

Answer 474

A type of transaction that can be optimized for InnoDB tables by eliminating some of the bookkeeping involved with creating a read view for each transaction. Can only perform non-locking read queries. It can be started explicitly with the syntax START TRANSACTION READ ONLY, or automatically under certain conditions. See Section 14.13.14, “Optimizing InnoDB Read-Only Transactions” for details.

Answer 475

A type of transaction that can be optimized for InnoDB tables by eliminating some of the bookkeeping involved with creating a read view for each transaction. Can only perform non-locking read queries. It can be started explicitly with the syntax START TRANSACTION READ ONLY, or automatically under certain conditions. See Section 14.13.14, “Optimizing InnoDB Read-Only Transactions” for details.

Answer 476

A type of transaction that can be optimized for InnoDB tables by eliminating some of the bookkeeping involved with creating a read view for each transaction. Can only perform non-locking read queries. It can be started explicitly with the syntax START TRANSACTION READ ONLY, or automatically under certain conditions. See Section 14.13.14, “Optimizing InnoDB Read-Only Transactions” for details.

Answer 477

A lock on an index record. For example, SELECT c1 FOR UPDATE FROM t WHERE c1 = 10; prevents any other transaction from inserting, updating, or deleting rows where the value of t.c1 is 10. Contrast with gap lock and next-key lock.

Answer 478

A lock on an index record. For example, SELECT c1 FOR UPDATE FROM t WHERE c1 = 10; prevents any other transaction from inserting, updating, or deleting rows where the value of t.c1 is 10. Contrast with gap lock and next-key lock.

Answer 479

A lock on an index record. For example, SELECT c1 FOR UPDATE FROM t WHERE c1 = 10; prevents any other transaction from inserting, updating, or deleting rows where the value of t.c1 is 10. Contrast with gap lock and next-key lock.

Answer 480

The data, in units of records, recorded in the redo log when DML statements make changes to InnoDB tables. It is used during crash recoveryto correct data written by incomplete transactions. The ever-increasing LSN value represents the cumulative amount of redo data that has passed through the redo log.

Answer 481

The data, in units of records, recorded in the redo log when DML statements make changes to InnoDB tables. It is used during crash recoveryto correct data written by incomplete transactions. The ever-increasing LSN value represents the cumulative amount of redo data that has passed through the redo log.

Answer 482

The data, in units of records, recorded in the redo log when DML statements make changes to InnoDB tables. It is used during crash recoveryto correct data written by incomplete transactions. The ever-increasing LSN value represents the cumulative amount of redo data that has passed through the redo log.

Answer 483

A disk-based data structure used during crash recovery, to correct data written by incomplete transactions. During normal operation, it encodes requests to change InnoDB table data, which result from SQL statements or low-level API calls through NoSQL interfaces. Modifications that did not finish updating the data files before an unexpected shutdown are replayed automatically. The redo log is physically represented as a set of files, typically named ib\_logfile0 and ib\_logfile1. The data in the redo log is encoded in terms of records affected; this data is collectively referred to as redo. The passage of data through the redo logs is represented by the ever-increasing LSN value. The original 4GB limit on maximum size for the redo log is raised to 512GB in MySQL 5.6.3. The disk layout of the redo log is influenced by the configuration options innodb\_log\_file\_size, innodb\_log\_group\_home\_dir, and (rarely) innodb\_log\_files\_in\_group. The performance of redo log operations is also affected by the log buffer, which is controlled by the innodb\_log\_buffer\_size configuration option.

Answer 484

A disk-based data structure used during crash recovery, to correct data written by incomplete transactions. During normal operation, it encodes requests to change InnoDB table data, which result from SQL statements or low-level API calls through NoSQL interfaces. Modifications that did not finish updating the data files before an unexpected shutdown are replayed automatically. The redo log is physically represented as a set of files, typically named ib\_logfile0 and ib\_logfile1. The data in the redo log is encoded in terms of records affected; this data is collectively referred to as redo. The passage of data through the redo logs is represented by the ever-increasing LSN value. The original 4GB limit on maximum size for the redo log is raised to 512GB in MySQL 5.6.3. The disk layout of the redo log is influenced by the configuration options innodb\_log\_file\_size, innodb\_log\_group\_home\_dir, and (rarely) innodb\_log\_files\_in\_group. The performance of redo log operations is also affected by the log buffer, which is controlled by the innodb\_log\_buffer\_size configuration option.

Answer 485

A disk-based data structure used during crash recovery, to correct data written by incomplete transactions. During normal operation, it encodes requests to change InnoDB table data, which result from SQL statements or low-level API calls through NoSQL interfaces. Modifications that did not finish updating the data files before an unexpected shutdown are replayed automatically. The redo log is physically represented as a set of files, typically named ib\_logfile0 and ib\_logfile1. The data in the redo log is encoded in terms of records affected; this data is collectively referred to as redo. The passage of data through the redo logs is represented by the ever-increasing LSN value. The original 4GB limit on maximum size for the redo log is raised to 512GB in MySQL 5.6.3. The disk layout of the redo log is influenced by the configuration options innodb\_log\_file\_size, innodb\_log\_group\_home\_dir, and (rarely) innodb\_log\_files\_in\_group. The performance of redo log operations is also affected by the log buffer, which is controlled by the innodb\_log\_buffer\_size configuration option.

Answer 486

The oldest InnoDB row format, available for tables using the Antelope file format. Prior to MySQL 5.0.3, it was the only row format available in InnoDB. In My SQL 5.0.3 and later, the default is compact row format. You can still specify redundant row format for compatibility with older InnoDB tables. For additional information about InnoDB REDUNDANT row format, see Section 14.9.4, “COMPACT and REDUNDANT Row Formats”.

Answer 487

The oldest InnoDB row format, available for tables using the Antelope file format. Prior to MySQL 5.0.3, it was the only row format available in InnoDB. In My SQL 5.0.3 and later, the default is compact row format. You can still specify redundant row format for compatibility with older InnoDB tables. For additional information about InnoDB REDUNDANT row format, see Section 14.9.4, “COMPACT and REDUNDANT Row Formats”.

Answer 488

The oldest InnoDB row format, available for tables using the Antelope file format. Prior to MySQL 5.0.3, it was the only row format available in InnoDB. In My SQL 5.0.3 and later, the default is compact row format. You can still specify redundant row format for compatibility with older InnoDB tables. For additional information about InnoDB REDUNDANT row format, see Section 14.9.4, “COMPACT and REDUNDANT Row Formats”.

Answer 489

The technique of maintaining data always in a consistent format, part of the ACID philosophy. In particular, data in different tables is kept consistent through the use of foreign key constraints, which can prevent changes from happening or automatically propagate those changes to all related tables. Related mechanisms include the unique constraint, which prevents duplicate values from being inserted by mistake, and theNOT NULL constraint, which prevents blank values from being inserted by mistake.

Answer 490

The technique of maintaining data always in a consistent format, part of the ACID philosophy. In particular, data in different tables is kept consistent through the use of foreign key constraints, which can prevent changes from happening or automatically propagate those changes to all related tables. Related mechanisms include the unique constraint, which prevents duplicate values from being inserted by mistake, and theNOT NULL constraint, which prevents blank values from being inserted by mistake.

Answer 491

The technique of maintaining data always in a consistent format, part of the ACID philosophy. In particular, data in different tables is kept consistent through the use of foreign key constraints, which can prevent changes from happening or automatically propagate those changes to all related tables. Related mechanisms include the unique constraint, which prevents duplicate values from being inserted by mistake, and theNOT NULL constraint, which prevents blank values from being inserted by mistake.

Answer 492

An important aspect of modern database systems. The database server encodes and enforces relationships such as one-to-one, one-to-many, many-to-one, and uniqueness. For example, a person might have zero, one, or many phone numbers in an address database; a single phone number might be associated with several family members. In a financial database, a person might be required to have exactly one taxpayer ID, and any taxpayer ID could only be associated with one person. The database server can use these relationships to prevent bad data from being inserted, and to find efficient ways to look up information. For example, if a value is declared to be unique, the server can stop searching as soon as the first match is found, and it can reject attempts to insert a second copy of the same value. At the database level, these relationships are expressed through SQL features such as columns within a table, unique and NOT NULL constraints, foreign keys, and different kinds of join operations. Complex relationships typically involve data split between more than one table. Often, the data is normalized, so that duplicate values in one-to-many relationships are stored only once. In a mathematical context, the relations within a database are derived from set theory. For example, the OR and AND operators of a WHERE clause represent the notions of union and intersection.

Answer 493

An important aspect of modern database systems. The database server encodes and enforces relationships such as one-to-one, one-to-many, many-to-one, and uniqueness. For example, a person might have zero, one, or many phone numbers in an address database; a single phone number might be associated with several family members. In a financial database, a person might be required to have exactly one taxpayer ID, and any taxpayer ID could only be associated with one person. The database server can use these relationships to prevent bad data from being inserted, and to find efficient ways to look up information. For example, if a value is declared to be unique, the server can stop searching as soon as the first match is found, and it can reject attempts to insert a second copy of the same value. At the database level, these relationships are expressed through SQL features such as columns within a table, unique and NOT NULL constraints, foreign keys, and different kinds of join operations. Complex relationships typically involve data split between more than one table. Often, the data is normalized, so that duplicate values in one-to-many relationships are stored only once. In a mathematical context, the relations within a database are derived from set theory. For example, the OR and AND operators of a WHERE clause represent the notions of union and intersection.

Answer 494

An important aspect of modern database systems. The database server encodes and enforces relationships such as one-to-one, one-to-many, many-to-one, and uniqueness. For example, a person might have zero, one, or many phone numbers in an address database; a single phone number might be associated with several family members. In a financial database, a person might be required to have exactly one taxpayer ID, and any taxpayer ID could only be associated with one person. The database server can use these relationships to prevent bad data from being inserted, and to find efficient ways to look up information. For example, if a value is declared to be unique, the server can stop searching as soon as the first match is found, and it can reject attempts to insert a second copy of the same value. At the database level, these relationships are expressed through SQL features such as columns within a table, unique and NOT NULL constraints, foreign keys, and different kinds of join operations. Complex relationships typically involve data split between more than one table. Often, the data is normalized, so that duplicate values in one-to-many relationships are stored only once. In a mathematical context, the relations within a database are derived from set theory. For example, the OR and AND operators of a WHERE clause represent the notions of union and intersection.

Answer 495

In the full-text search feature, a number signifying the similarity between the search string and the data in the FULLTEXT index. For example, when you search for a single word, that word is typically more relevant for a row where if it occurs several times in the text than a row where it appears only once.

Answer 496

In the full-text search feature, a number signifying the similarity between the search string and the data in the FULLTEXT index. For example, when you search for a single word, that word is typically more relevant for a row where if it occurs several times in the text than a row where it appears only once.

Answer 497

In the full-text search feature, a number signifying the similarity between the search string and the data in the FULLTEXT index. For example, when you search for a single word, that word is typically more relevant for a row where if it occurs several times in the text than a row where it appears only once.

Answer 498

The default isolation level for InnoDB. It prevents any rows that are queried from being changed by other transactions, thus blocking non-repeatable reads but not phantom reads. It uses a moderately strict locking strategy so that all queries within a transaction see data from the same snapshot, that is, the data as it was at the time the transaction started. When a transaction with this isolation level performs UPDATE ... WHERE, DELETE ... WHERE, SELECT ... FOR UPDATE, and LOCK IN SHARE MODE operations, other transactions might have to wait.

Answer 499

The default isolation level for InnoDB. It prevents any rows that are queried from being changed by other transactions, thus blocking non-repeatable reads but not phantom reads. It uses a moderately strict locking strategy so that all queries within a transaction see data from the same snapshot, that is, the data as it was at the time the transaction started. When a transaction with this isolation level performs UPDATE ... WHERE, DELETE ... WHERE, SELECT ... FOR UPDATE, and LOCK IN SHARE MODE operations, other transactions might have to wait.

Answer 500

The default isolation level for InnoDB. It prevents any rows that are queried from being changed by other transactions, thus blocking non-repeatable reads but not phantom reads. It uses a moderately strict locking strategy so that all queries within a transaction see data from the same snapshot, that is, the data as it was at the time the transaction started. When a transaction with this isolation level performs UPDATE ... WHERE, DELETE ... WHERE, SELECT ... FOR UPDATE, and LOCK IN SHARE MODE operations, other transactions might have to wait.

Answer 501

The practice of sending changes from a master database, to one or more slave databases, so that all databases have the same data. This technique has a wide range of uses, such as load-balancing for better scalability, disaster recovery, and testing software upgrades and configuration changes. The changes can be sent between the database by methods called row-based replication and statement-based replication.

Answer 502

The practice of sending changes from a master database, to one or more slave databases, so that all databases have the same data. This technique has a wide range of uses, such as load-balancing for better scalability, disaster recovery, and testing software upgrades and configuration changes. The changes can be sent between the database by methods called row-based replication and statement-based replication.

Answer 503

The practice of sending changes from a master database, to one or more slave databases, so that all databases have the same data. This technique has a wide range of uses, such as load-balancing for better scalability, disaster recovery, and testing software upgrades and configuration changes. The changes can be sent between the database by methods called row-based replication and statement-based replication.

Answer 504

The process of putting a set of backup files from the MySQL Enterprise Backup product in place for use by MySQL. This operation can be performed to fix a corrupted database, to return to some earlier point in time, or (in a replication context) to set up a new slave database. In theMySQL Enterprise Backup product, this operation is performed by the copy-back option of the mysqlbackup command.

Answer 505

The process of putting a set of backup files from the MySQL Enterprise Backup product in place for use by MySQL. This operation can be performed to fix a corrupted database, to return to some earlier point in time, or (in a replication context) to set up a new slave database. In theMySQL Enterprise Backup product, this operation is performed by the copy-back option of the mysqlbackup command.

Answer 506

The process of putting a set of backup files from the MySQL Enterprise Backup product in place for use by MySQL. This operation can be performed to fix a corrupted database, to return to some earlier point in time, or (in a replication context) to set up a new slave database. In theMySQL Enterprise Backup product, this operation is performed by the copy-back option of the mysqlbackup command.

Answer 507

A SQL statement that ends a transaction, undoing any changes made by the transaction. It is the opposite of commit, which makes permanent any changes made in the transaction. By default, MySQL uses the autocommit setting, which automatically issues a commit following each SQL statement. You must change this setting before you can use the rollback technique.

Answer 508

A SQL statement that ends a transaction, undoing any changes made by the transaction. It is the opposite of commit, which makes permanent any changes made in the transaction. By default, MySQL uses the autocommit setting, which automatically issues a commit following each SQL statement. You must change this setting before you can use the rollback technique.

Answer 509

A SQL statement that ends a transaction, undoing any changes made by the transaction. It is the opposite of commit, which makes permanent any changes made in the transaction. By default, MySQL uses the autocommit setting, which automatically issues a commit following each SQL statement. You must change this setting before you can use the rollback technique.

Answer 510

The storage area containing the undo log, part of the system tablespace.

Answer 511

The storage area containing the undo log, part of the system tablespace.

Answer 512

The storage area containing the undo log, part of the system tablespace.

Answer 513

The logical data structure defined by a set of columns. A set of rows makes up a table. Within InnoDB data files, each page can contain one or more rows. Although InnoDB uses the term row format for consistency with MySQL syntax, the row format is a property of each table and applies to all rows in that table.

Answer 514

The logical data structure defined by a set of columns. A set of rows makes up a table. Within InnoDB data files, each page can contain one or more rows. Although InnoDB uses the term row format for consistency with MySQL syntax, the row format is a property of each table and applies to all rows in that table.

Answer 515

The logical data structure defined by a set of columns. A set of rows makes up a table. Within InnoDB data files, each page can contain one or more rows. Although InnoDB uses the term row format for consistency with MySQL syntax, the row format is a property of each table and applies to all rows in that table.

Answer 516

The disk storage format for a row from an InnoDB table. As InnoDB gains new capabilities such as compression, new row formats are introduced to support the resulting improvements in storage efficiency and performance. Each table has its own row format, specified through the ROW\_FORMAT option. To see the row format for each InnoDB table, issue the command SHOW TABLE STATUS. Because all the tables in the system tablespace share the same row format, to take advantage of other row formats typically requires setting the innodb\_file\_per\_table option, so that each table is stored in a separate tablespace.

Answer 517

The disk storage format for a row from an InnoDB table. As InnoDB gains new capabilities such as compression, new row formats are introduced to support the resulting improvements in storage efficiency and performance. Each table has its own row format, specified through the ROW\_FORMAT option. To see the row format for each InnoDB table, issue the command SHOW TABLE STATUS. Because all the tables in the system tablespace share the same row format, to take advantage of other row formats typically requires setting the innodb\_file\_per\_table option, so that each table is stored in a separate tablespace.

Answer 518

The disk storage format for a row from an InnoDB table. As InnoDB gains new capabilities such as compression, new row formats are introduced to support the resulting improvements in storage efficiency and performance. Each table has its own row format, specified through the ROW\_FORMAT option. To see the row format for each InnoDB table, issue the command SHOW TABLE STATUS. Because all the tables in the system tablespace share the same row format, to take advantage of other row formats typically requires setting the innodb\_file\_per\_table option, so that each table is stored in a separate tablespace.

Answer 519

A lock that prevents a row from being accessed in an incompatible way by another transaction. Other rows in the same table can be freely written to by other transactions. This is the type of locking done by DML operations on InnoDB tables. Contrast with table locks used by MyISAM, or during DDL operations on InnoDB tables that cannot be done with online DDL; those locks block concurrent access to the table.

Answer 520

A lock that prevents a row from being accessed in an incompatible way by another transaction. Other rows in the same table can be freely written to by other transactions. This is the type of locking done by DML operations on InnoDB tables. Contrast with table locks used by MyISAM, or during DDL operations on InnoDB tables that cannot be done with online DDL; those locks block concurrent access to the table.

Answer 521

A lock that prevents a row from being accessed in an incompatible way by another transaction. Other rows in the same table can be freely written to by other transactions. This is the type of locking done by DML operations on InnoDB tables. Contrast with table locks used by MyISAM, or during DDL operations on InnoDB tables that cannot be done with online DDL; those locks block concurrent access to the table.

Answer 522

A form of replication where events are propagated from the master server specifying how to change individual rows on the slave server. It is safe to use for all settings of the innodb\_autoinc\_lock\_mode option.

Answer 523

A form of replication where events are propagated from the master server specifying how to change individual rows on the slave server. It is safe to use for all settings of the innodb\_autoinc\_lock\_mode option.

Answer 524

A form of replication where events are propagated from the master server specifying how to change individual rows on the slave server. It is safe to use for all settings of the innodb\_autoinc\_lock\_mode option.

Answer 525

The locking mechanism used for InnoDB tables, relying on row locks rather than table locks. Multiple transactions can modify the same table concurrently. Only if two transactions try to modify the same row does one of the transactions wait for the other to complete (and release its row locks).

Answer 526

The locking mechanism used for InnoDB tables, relying on row locks rather than table locks. Multiple transactions can modify the same table concurrently. Only if two transactions try to modify the same row does one of the transactions wait for the other to complete (and release its row locks).

Answer 527

The locking mechanism used for InnoDB tables, relying on row locks rather than table locks. Multiple transactions can modify the same table concurrently. Only if two transactions try to modify the same row does one of the transactions wait for the other to complete (and release its row locks).

Answer 528

The low-level object that InnoDB uses to represent and enforce shared-access locks to internal in-memory data structures following certain rules. Contrast with mutexes, which InnoDB uses to represent and enforce exclusive access to internal in-memory data structures. Mutexes and rw-locks are known collectively as latches. rw-lock types include s-locks (shared locks), x-locks (exclusive locks), and sx-locks (shared-exclusive locks). An s-lock provides read access to a common resource. An x-lock provides write access to a common resource while not permitting inconsistent reads by other threads. An sx-lock provides write access to a common resource while permitting inconsistent reads by other threads. sx-locks were introduced in MySQL 5.7 to optimize concurrency and improve scalability for read-write workloads. The following matrix summarizes rw-lock type compatibility. S SX X S Compatible Compatible Conflict SX Compatible Conflict Conflict X Conflict Conflict Conflict

Answer 529

The low-level object that InnoDB uses to represent and enforce shared-access locks to internal in-memory data structures following certain rules. Contrast with mutexes, which InnoDB uses to represent and enforce exclusive access to internal in-memory data structures. Mutexes and rw-locks are known collectively as latches. rw-lock types include s-locks (shared locks), x-locks (exclusive locks), and sx-locks (shared-exclusive locks). An s-lock provides read access to a common resource. An x-lock provides write access to a common resource while not permitting inconsistent reads by other threads. An sx-lock provides write access to a common resource while permitting inconsistent reads by other threads. sx-locks were introduced in MySQL 5.7 to optimize concurrency and improve scalability for read-write workloads. The following matrix summarizes rw-lock type compatibility. S SX X S Compatible Compatible Conflict SX Compatible Conflict Conflict X Conflict Conflict Conflict

Answer 530

The low-level object that InnoDB uses to represent and enforce shared-access locks to internal in-memory data structures following certain rules. Contrast with mutexes, which InnoDB uses to represent and enforce exclusive access to internal in-memory data structures. Mutexes and rw-locks are known collectively as latches. rw-lock types include s-locks (shared locks), x-locks (exclusive locks), and sx-locks (shared-exclusive locks). An s-lock provides read access to a common resource. An x-lock provides write access to a common resource while not permitting inconsistent reads by other threads. An sx-lock provides write access to a common resource while permitting inconsistent reads by other threads. sx-locks were introduced in MySQL 5.7 to optimize concurrency and improve scalability for read-write workloads. The following matrix summarizes rw-lock type compatibility. S SX X S Compatible Compatible Conflict SX Compatible Conflict Conflict X Conflict Conflict Conflict

Answer 531

The ability to add more work and issue more simultaneous requests to a system, without a sudden drop in performance due to exceeding the limits of system capacity. Software architecture, hardware configuration, application coding, and type of workload all play a part in scalability. When the system reaches its maximum capacity, popular techniques for increasing scalability are scale up (increasing the capacity of existing hardware or software) and scale out (adding new servers and more instances of MySQL). Often paired with availability as critical aspects of a large-scale deployment.

Answer 532

The ability to add more work and issue more simultaneous requests to a system, without a sudden drop in performance due to exceeding the limits of system capacity. Software architecture, hardware configuration, application coding, and type of workload all play a part in scalability. When the system reaches its maximum capacity, popular techniques for increasing scalability are scale up (increasing the capacity of existing hardware or software) and scale out (adding new servers and more instances of MySQL). Often paired with availability as critical aspects of a large-scale deployment.

Answer 533

The ability to add more work and issue more simultaneous requests to a system, without a sudden drop in performance due to exceeding the limits of system capacity. Software architecture, hardware configuration, application coding, and type of workload all play a part in scalability. When the system reaches its maximum capacity, popular techniques for increasing scalability are scale up (increasing the capacity of existing hardware or software) and scale out (adding new servers and more instances of MySQL). Often paired with availability as critical aspects of a large-scale deployment.

Answer 534

A technique for increasing scalability by adding new servers and more instances of MySQL. For example, setting up replication, MySQL Cluster, connection pooling, or other features that spread work across a group of servers. Contrast with scale up.

Answer 535

A technique for increasing scalability by adding new servers and more instances of MySQL. For example, setting up replication, MySQL Cluster, connection pooling, or other features that spread work across a group of servers. Contrast with scale up.

Answer 536

A technique for increasing scalability by adding new servers and more instances of MySQL. For example, setting up replication, MySQL Cluster, connection pooling, or other features that spread work across a group of servers. Contrast with scale up.

Answer 537

A technique for increasing scalability by increasing the capacity of existing hardware or software. For example, increasing the memory on a server and adjusting memory-related parameters such as innodb\_buffer\_pool\_size andinnodb\_buffer\_pool\_instances. Contrast with scale out.

Answer 538

A technique for increasing scalability by increasing the capacity of existing hardware or software. For example, increasing the memory on a server and adjusting memory-related parameters such as innodb\_buffer\_pool\_size andinnodb\_buffer\_pool\_instances. Contrast with scale out.

Answer 539

A technique for increasing scalability by increasing the capacity of existing hardware or software. For example, increasing the memory on a server and adjusting memory-related parameters such as innodb\_buffer\_pool\_size andinnodb\_buffer\_pool\_instances. Contrast with scale out.

Answer 540

Conceptually, a schema is a set of interrelated database objects, such as tables, table columns, data types of the columns, indexes, foreign keys, and so on. These objects are connected through SQL syntax, because the columns make up the tables, the foreign keys refer to tables and columns, and so on. Ideally, they are also connected logically, working together as part of a unified application or flexible framework. For example, the information\_schema and performance\_schema databases use "schema" in their names to emphasize the close relationships between the tables and columns they contain. In MySQL, physically, a schema is synonymous with a database. You can substitute the keyword SCHEMA instead of DATABASE in MySQL SQL syntax, for example using CREATE SCHEMA instead of CREATE DATABASE. Some other database products draw a distinction. For example, in the Oracle Database product, a schema represents only a part of a database: the tables and other objects owned by a single user.

Answer 541

Conceptually, a schema is a set of interrelated database objects, such as tables, table columns, data types of the columns, indexes, foreign keys, and so on. These objects are connected through SQL syntax, because the columns make up the tables, the foreign keys refer to tables and columns, and so on. Ideally, they are also connected logically, working together as part of a unified application or flexible framework. For example, the information\_schema and performance\_schema databases use "schema" in their names to emphasize the close relationships between the tables and columns they contain. In MySQL, physically, a schema is synonymous with a database. You can substitute the keyword SCHEMA instead of DATABASE in MySQL SQL syntax, for example using CREATE SCHEMA instead of CREATE DATABASE. Some other database products draw a distinction. For example, in the Oracle Database product, a schema represents only a part of a database: the tables and other objects owned by a single user.

Answer 542

Conceptually, a schema is a set of interrelated database objects, such as tables, table columns, data types of the columns, indexes, foreign keys, and so on. These objects are connected through SQL syntax, because the columns make up the tables, the foreign keys refer to tables and columns, and so on. Ideally, they are also connected logically, working together as part of a unified application or flexible framework. For example, the information\_schema and performance\_schema databases use "schema" in their names to emphasize the close relationships between the tables and columns they contain. In MySQL, physically, a schema is synonymous with a database. You can substitute the keyword SCHEMA instead of DATABASE in MySQL SQL syntax, for example using CREATE SCHEMA instead of CREATE DATABASE. Some other database products draw a distinction. For example, in the Oracle Database product, a schema represents only a part of a database: the tables and other objects owned by a single user.

Answer 543

In MySQL, full-text search queries use a special kind of index, the FULLTEXT index. In MySQL 5.6.4 and up, InnoDB and MyISAM tables both support FULLTEXT indexes; formerly, these indexes were only available for MyISAM tables.

Answer 544

In MySQL, full-text search queries use a special kind of index, the FULLTEXT index. In MySQL 5.6.4 and up, InnoDB and MyISAM tables both support FULLTEXT indexes; formerly, these indexes were only available for MyISAM tables.

Answer 545

In MySQL, full-text search queries use a special kind of index, the FULLTEXT index. In MySQL 5.6.4 and up, InnoDB and MyISAM tables both support FULLTEXT indexes; formerly, these indexes were only available for MyISAM tables.

Answer 546

A type of InnoDB index that represents a subset of table columns. An InnoDB table can have zero, one, or many secondary indexes. (Contrast with the clustered index, which is required for each InnoDB table, and stores the data for all the table columns.) A secondary index can be used to satisfy queries that only require values from the indexed columns. For more complex queries, it can be used to identify the relevant rows in the table, which are then retrieved through lookups using the clustered index. Creating and dropping secondary indexes has traditionally involved significant overhead from copying all the data in the InnoDB table. The fast index creation feature of the InnoDB Plugin makes both CREATE INDEX and DROP INDEX statements much faster for InnoDB secondary indexes.

Answer 547

A type of InnoDB index that represents a subset of table columns. An InnoDB table can have zero, one, or many secondary indexes. (Contrast with the clustered index, which is required for each InnoDB table, and stores the data for all the table columns.) A secondary index can be used to satisfy queries that only require values from the indexed columns. For more complex queries, it can be used to identify the relevant rows in the table, which are then retrieved through lookups using the clustered index. Creating and dropping secondary indexes has traditionally involved significant overhead from copying all the data in the InnoDB table. The fast index creation feature of the InnoDB Plugin makes both CREATE INDEX and DROP INDEX statements much faster for InnoDB secondary indexes.

Answer 548

A type of InnoDB index that represents a subset of table columns. An InnoDB table can have zero, one, or many secondary indexes. (Contrast with the clustered index, which is required for each InnoDB table, and stores the data for all the table columns.) A secondary index can be used to satisfy queries that only require values from the indexed columns. For more complex queries, it can be used to identify the relevant rows in the table, which are then retrieved through lookups using the clustered index. Creating and dropping secondary indexes has traditionally involved significant overhead from copying all the data in the InnoDB table. The fast index creation feature of the InnoDB Plugin makes both CREATE INDEX and DROP INDEX statements much faster for InnoDB secondary indexes.

Answer 549

A division within an InnoDB tablespace. If a tablespace is analogous to a directory, the segments are analogous to files within that directory. A segment can grow. New segments can be created. For example, within a file-per-table tablespace, the table data is in one segment and each associated index is in its own segment. The system tablespace contains many different segments, because it can hold many tables and their associated indexes. The system tablespace also includes up to 128 rollback segments making up the undo log. Segments grow and shrink as data is inserted and deleted. When a segment needs more room, it is extended by one extent (1 megabyte) at a time. Similarly, a segment releases one extent's worth of space when all the data in that extent is no longer needed.

Answer 550

A division within an InnoDB tablespace. If a tablespace is analogous to a directory, the segments are analogous to files within that directory. A segment can grow. New segments can be created. For example, within a file-per-table tablespace, the table data is in one segment and each associated index is in its own segment. The system tablespace contains many different segments, because it can hold many tables and their associated indexes. The system tablespace also includes up to 128 rollback segments making up the undo log. Segments grow and shrink as data is inserted and deleted. When a segment needs more room, it is extended by one extent (1 megabyte) at a time. Similarly, a segment releases one extent's worth of space when all the data in that extent is no longer needed.

Answer 551

A division within an InnoDB tablespace. If a tablespace is analogous to a directory, the segments are analogous to files within that directory. A segment can grow. New segments can be created. For example, within a file-per-table tablespace, the table data is in one segment and each associated index is in its own segment. The system tablespace contains many different segments, because it can hold many tables and their associated indexes. The system tablespace also includes up to 128 rollback segments making up the undo log. Segments grow and shrink as data is inserted and deleted. When a segment needs more room, it is extended by one extent (1 megabyte) at a time. Similarly, a segment releases one extent's worth of space when all the data in that extent is no longer needed.

Answer 552

A property of data distribution, the number of distinct values in a column (its cardinality) divided by the number of records in the table. High selectivity means that the column values are relatively unique, and can retrieved efficiently through an index. If you (or the query optimizer) can predict that a test in a WHERE clause only matches a small number (or proportion) of rows in a table, the overall query tends to be efficient if it evaluates that test first, using an index.

Answer 553

A property of data distribution, the number of distinct values in a column (its cardinality) divided by the number of records in the table. High selectivity means that the column values are relatively unique, and can retrieved efficiently through an index. If you (or the query optimizer) can predict that a test in a WHERE clause only matches a small number (or proportion) of rows in a table, the overall query tends to be efficient if it evaluates that test first, using an index.

Answer 554

A property of data distribution, the number of distinct values in a column (its cardinality) divided by the number of records in the table. High selectivity means that the column values are relatively unique, and can retrieved efficiently through an index. If you (or the query optimizer) can predict that a test in a WHERE clause only matches a small number (or proportion) of rows in a table, the overall query tends to be efficient if it evaluates that test first, using an index.

Answer 555

A type of read operation used for UPDATE statements, that is a combination of read committed and consistent read. When an UPDATEstatement examines a row that is already locked, InnoDB returns the latest committed version to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE. If the row matches (must be updated), MySQL reads the row again, and this time InnoDB either locks it or waits for a lock on it. This type of read operation can only happen when the transaction has the read committed isolation level, or when the innodb\_locks\_unsafe\_for\_binlog option is enabled.

Answer 556

A type of read operation used for UPDATE statements, that is a combination of read committed and consistent read. When an UPDATEstatement examines a row that is already locked, InnoDB returns the latest committed version to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE. If the row matches (must be updated), MySQL reads the row again, and this time InnoDB either locks it or waits for a lock on it. This type of read operation can only happen when the transaction has the read committed isolation level, or when the innodb\_locks\_unsafe\_for\_binlog option is enabled.

Answer 557

A type of read operation used for UPDATE statements, that is a combination of read committed and consistent read. When an UPDATEstatement examines a row that is already locked, InnoDB returns the latest committed version to MySQL so that MySQL can determine whether the row matches the WHERE condition of the UPDATE. If the row matches (must be updated), MySQL reads the row again, and this time InnoDB either locks it or waits for a lock on it. This type of read operation can only happen when the transaction has the read committed isolation level, or when the innodb\_locks\_unsafe\_for\_binlog option is enabled.

Answer 558

The isolation level that uses the most conservative locking strategy, to prevent any other transactions from inserting or changing data that was read by this transaction, until it is finished. This way, the same query can be run over and over within a transaction, and be certain to retrieve the same set of results each time. Any attempt to change data that was committed by another transaction since the start of the current transaction, cause the current transaction to wait. This is the default isolation level specified by the SQL standard. In practice, this degree of strictness is rarely needed, so the default isolation level for InnoDB is the next most strict, repeatable read.

Answer 559

The isolation level that uses the most conservative locking strategy, to prevent any other transactions from inserting or changing data that was read by this transaction, until it is finished. This way, the same query can be run over and over within a transaction, and be certain to retrieve the same set of results each time. Any attempt to change data that was committed by another transaction since the start of the current transaction, cause the current transaction to wait. This is the default isolation level specified by the SQL standard. In practice, this degree of strictness is rarely needed, so the default isolation level for InnoDB is the next most strict, repeatable read.

Answer 560

The isolation level that uses the most conservative locking strategy, to prevent any other transactions from inserting or changing data that was read by this transaction, until it is finished. This way, the same query can be run over and over within a transaction, and be certain to retrieve the same set of results each time. Any attempt to change data that was committed by another transaction since the start of the current transaction, cause the current transaction to wait. This is the default isolation level specified by the SQL standard. In practice, this degree of strictness is rarely needed, so the default isolation level for InnoDB is the next most strict, repeatable read.

Answer 561

A type of program that runs continuously, waiting to receive and act upon requests from another program (the client). Because often an entire computer is dedicated to running one or more server programs (such as a database server, a web server, an application server, or some combination of these), the term server can also refer to the computer that runs the server software.

Answer 562

A type of program that runs continuously, waiting to receive and act upon requests from another program (the client). Because often an entire computer is dedicated to running one or more server programs (such as a database server, a web server, an application server, or some combination of these), the term server can also refer to the computer that runs the server software.

Answer 563

A type of program that runs continuously, waiting to receive and act upon requests from another program (the client). Because often an entire computer is dedicated to running one or more server programs (such as a database server, a web server, an application server, or some combination of these), the term server can also refer to the computer that runs the server software.

Answer 564

A kind of lock that allows other transactions to read the locked object, and to also acquire other shared locks on it, but not to write to it. The opposite of exclusive lock.

Answer 565

A kind of lock that allows other transactions to read the locked object, and to also acquire other shared locks on it, but not to write to it. The opposite of exclusive lock.

Answer 566

A kind of lock that allows other transactions to read the locked object, and to also acquire other shared locks on it, but not to write to it. The opposite of exclusive lock.

Answer 567

Another way of referring to the system tablespace.

Answer 568

Another way of referring to the system tablespace.

Answer 569

Another way of referring to the system tablespace.

Answer 570

The process of flushing to disk all dirty buffer pool pages whose redo entries are contained in certain portion of the redo log. Occurs before InnoDB reuses a portion of a log file; the log files are used in a circular fashion. Typically occurs with write-intensive workloads.

Answer 571

The process of flushing to disk all dirty buffer pool pages whose redo entries are contained in certain portion of the redo log. Occurs before InnoDB reuses a portion of a log file; the log files are used in a circular fashion. Typically occurs with write-intensive workloads.

Answer 572

The process of flushing to disk all dirty buffer pool pages whose redo entries are contained in certain portion of the redo log. Occurs before InnoDB reuses a portion of a log file; the log files are used in a circular fashion. Typically occurs with write-intensive workloads.

Answer 573

The process of stopping the MySQL server. By default, this process does cleanup operations for InnoDB tables, so it can slow to shut down, but fast to start up later. If you skip the cleanup operations, it is fast to shut down but must do the cleanup during the next restart. The shutdown mode is controlled by the innodb\_fast\_shutdown option.

Answer 574

The process of stopping the MySQL server. By default, this process does cleanup operations for InnoDB tables, so it can slow to shut down, but fast to start up later. If you skip the cleanup operations, it is fast to shut down but must do the cleanup during the next restart. The shutdown mode is controlled by the innodb\_fast\_shutdown option.

Answer 575

The process of stopping the MySQL server. By default, this process does cleanup operations for InnoDB tables, so it can slow to shut down, but fast to start up later. If you skip the cleanup operations, it is fast to shut down but must do the cleanup during the next restart. The shutdown mode is controlled by the innodb\_fast\_shutdown option.

Answer 576

Frequently shortened to "slave". A database server machine in a replication scenario that receives changes from another server (the master) and applies those same changes. Thus it maintains the same contents as the master, although it might lag somewhat behind. In MySQL, slave servers are commonly used in disaster recovery, to take the place of a master servers that fails. They are also commonly used for testing software upgrades and new settings, to ensure that database configuration changes do not cause problems with performance or reliability. Slave servers typically have high workloads, because they process all the DML (write) operations relayed from the master, as well as user queries. To ensure that slave servers can apply changes from the master fast enough, they frequently have fast I/O devices and sufficient CPU and memory to run multiple database instances on the same slave server. For example, the master server might use hard drive storage while the slave servers use SSDs.

Answer 577

Frequently shortened to "slave". A database server machine in a replication scenario that receives changes from another server (the master) and applies those same changes. Thus it maintains the same contents as the master, although it might lag somewhat behind. In MySQL, slave servers are commonly used in disaster recovery, to take the place of a master servers that fails. They are also commonly used for testing software upgrades and new settings, to ensure that database configuration changes do not cause problems with performance or reliability. Slave servers typically have high workloads, because they process all the DML (write) operations relayed from the master, as well as user queries. To ensure that slave servers can apply changes from the master fast enough, they frequently have fast I/O devices and sufficient CPU and memory to run multiple database instances on the same slave server. For example, the master server might use hard drive storage while the slave servers use SSDs.

Answer 578

Frequently shortened to "slave". A database server machine in a replication scenario that receives changes from another server (the master) and applies those same changes. Thus it maintains the same contents as the master, although it might lag somewhat behind. In MySQL, slave servers are commonly used in disaster recovery, to take the place of a master servers that fails. They are also commonly used for testing software upgrades and new settings, to ensure that database configuration changes do not cause problems with performance or reliability. Slave servers typically have high workloads, because they process all the DML (write) operations relayed from the master, as well as user queries. To ensure that slave servers can apply changes from the master fast enough, they frequently have fast I/O devices and sufficient CPU and memory to run multiple database instances on the same slave server. For example, the master server might use hard drive storage while the slave servers use SSDs.

Answer 579

A type of log used for performance tuning of SQL statements processed by the MySQL server. The log information is stored in a file. You must enable this feature to use it. You control which categories of "slow" SQL statements are logged. For more information, see Section 5.2.5, “The Slow Query Log”.

Answer 580

A type of log used for performance tuning of SQL statements processed by the MySQL server. The log information is stored in a file. You must enable this feature to use it. You control which categories of "slow" SQL statements are logged. For more information, see Section 5.2.5, “The Slow Query Log”.

Answer 581

A type of log used for performance tuning of SQL statements processed by the MySQL server. The log information is stored in a file. You must enable this feature to use it. You control which categories of "slow" SQL statements are logged. For more information, see Section 5.2.5, “The Slow Query Log”.

Answer 582

A type of shutdown that does additional InnoDB flushing operations before completing. Also known as a clean shutdown. Specified by the configuration parameter innodb\_fast\_shutdown=0 or the command SET GLOBAL innodb\_fast\_shutdown=0;. Although the shutdown itself can take longer, that time will be saved on the subsequent startup.

Answer 583

A type of shutdown that does additional InnoDB flushing operations before completing. Also known as a clean shutdown. Specified by the configuration parameter innodb\_fast\_shutdown=0 or the command SET GLOBAL innodb\_fast\_shutdown=0;. Although the shutdown itself can take longer, that time will be saved on the subsequent startup.

Answer 584

A type of shutdown that does additional InnoDB flushing operations before completing. Also known as a clean shutdown. Specified by the configuration parameter innodb\_fast\_shutdown=0 or the command SET GLOBAL innodb\_fast\_shutdown=0;. Although the shutdown itself can take longer, that time will be saved on the subsequent startup.

Answer 585

A representation of data at a particular time, which remains the same even as changes are committed by other transactions. Used by certainisolation levels to allow consistent reads.

Answer 586

A representation of data at a particular time, which remains the same even as changes are committed by other transactions. Used by certainisolation levels to allow consistent reads.

Answer 587

A representation of data at a particular time, which remains the same even as changes are committed by other transactions. Used by certainisolation levels to allow consistent reads.

Answer 588

An identifier used to uniquely identify an InnoDB tablespace within a MySQL instance. The space ID for the system tablespace is always zero; this same ID applies to all tables within the system tablespace. Each tablespace file created in file-per-table mode also has its own space ID. Prior to MySQL 5.6, this hardcoded value presented difficulties in moving InnoDB tablespace files between MySQL instances. Starting in MySQL 5.6, you can copy tablespace files between instances by using the transportable tablespace feature involving the statements FLUSH TABLES ... FOR EXPORT, ALTER TABLE ... DISCARD TABLESPACE, and ALTER TABLE ... IMPORT TABLESPACE. The information needed to adjust the space ID is conveyed in the .cfg file which you copy along with the tablespace. See Section 14.5.5, “Copying Tablespaces to Another Server (Transportable Tablespaces)” for details.

Answer 589

An identifier used to uniquely identify an InnoDB tablespace within a MySQL instance. The space ID for the system tablespace is always zero; this same ID applies to all tables within the system tablespace. Each tablespace file created in file-per-table mode also has its own space ID. Prior to MySQL 5.6, this hardcoded value presented difficulties in moving InnoDB tablespace files between MySQL instances. Starting in MySQL 5.6, you can copy tablespace files between instances by using the transportable tablespace feature involving the statements FLUSH TABLES ... FOR EXPORT, ALTER TABLE ... DISCARD TABLESPACE, and ALTER TABLE ... IMPORT TABLESPACE. The information needed to adjust the space ID is conveyed in the .cfg file which you copy along with the tablespace. See Section 14.5.5, “Copying Tablespaces to Another Server (Transportable Tablespaces)” for details.

Answer 590

An identifier used to uniquely identify an InnoDB tablespace within a MySQL instance. The space ID for the system tablespace is always zero; this same ID applies to all tables within the system tablespace. Each tablespace file created in file-per-table mode also has its own space ID. Prior to MySQL 5.6, this hardcoded value presented difficulties in moving InnoDB tablespace files between MySQL instances. Starting in MySQL 5.6, you can copy tablespace files between instances by using the transportable tablespace feature involving the statements FLUSH TABLES ... FOR EXPORT, ALTER TABLE ... DISCARD TABLESPACE, and ALTER TABLE ... IMPORT TABLESPACE. The information needed to adjust the space ID is conveyed in the .cfg file which you copy along with the tablespace. See Section 14.5.5, “Copying Tablespaces to Another Server (Transportable Tablespaces)” for details.

Answer 591

A type of wait operation that continuously tests whether a resource becomes available. This technique is used for resources that are typically held only for brief periods, where it is more efficient to wait in a "busy loop" than to put the thread to sleep and perform a context switch. If the resource does not become available within a short time, the spin loop ceases and another wait technique is used.

Answer 592

A type of wait operation that continuously tests whether a resource becomes available. This technique is used for resources that are typically held only for brief periods, where it is more efficient to wait in a "busy loop" than to put the thread to sleep and perform a context switch. If the resource does not become available within a short time, the spin loop ceases and another wait technique is used.

Answer 593

A type of wait operation that continuously tests whether a resource becomes available. This technique is used for resources that are typically held only for brief periods, where it is more efficient to wait in a "busy loop" than to put the thread to sleep and perform a context switch. If the resource does not become available within a short time, the spin loop ceases and another wait technique is used.

Answer 594

The Structured Query Language that is standard for performing database operations. Often divided into the categories DDL, DML, and queries. MySQL includes some additional statement categories such as replication. See Chapter 9, Language Structure for the building blocks of SQL syntax, Chapter 11, Data Types for the data types to use for MySQL table columns, Chapter 13, SQL Statement Syntax for details about SQL statements and their associated categories, and Chapter 12, Functions and Operators for standard and MySQL-specific functions to use in queries.

Answer 595

The Structured Query Language that is standard for performing database operations. Often divided into the categories DDL, DML, and queries. MySQL includes some additional statement categories such as replication. See Chapter 9, Language Structure for the building blocks of SQL syntax, Chapter 11, Data Types for the data types to use for MySQL table columns, Chapter 13, SQL Statement Syntax for details about SQL statements and their associated categories, and Chapter 12, Functions and Operators for standard and MySQL-specific functions to use in queries.

Answer 596

The Structured Query Language that is standard for performing database operations. Often divided into the categories DDL, DML, and queries. MySQL includes some additional statement categories such as replication. See Chapter 9, Language Structure for the building blocks of SQL syntax, Chapter 11, Data Types for the data types to use for MySQL table columns, Chapter 13, SQL Statement Syntax for details about SQL statements and their associated categories, and Chapter 12, Functions and Operators for standard and MySQL-specific functions to use in queries.

Answer 597

Acronym for "solid-state drive". A type of storage device with different performance characteristics than a traditional hard disk drive (HDD): smaller storage capacity, faster for random reads, no moving parts, and with a number of considerations affecting write performance. Its performance characteristics can influence the throughput of a disk-bound workload.

Answer 598

Acronym for "solid-state drive". A type of storage device with different performance characteristics than a traditional hard disk drive (HDD): smaller storage capacity, faster for random reads, no moving parts, and with a number of considerations affecting write performance. Its performance characteristics can influence the throughput of a disk-bound workload.

Answer 599

Acronym for "solid-state drive". A type of storage device with different performance characteristics than a traditional hard disk drive (HDD): smaller storage capacity, faster for random reads, no moving parts, and with a number of considerations affecting write performance. Its performance characteristics can influence the throughput of a disk-bound workload.

Answer 600

The process of starting the MySQL server. Typically done by one of the programs listed in Section 4.3, “MySQL Server and Server-Startup Programs”. The opposite of shutdown.

Answer 601

The process of starting the MySQL server. Typically done by one of the programs listed in Section 4.3, “MySQL Server and Server-Startup Programs”. The opposite of shutdown.

Answer 602

The process of starting the MySQL server. Typically done by one of the programs listed in Section 4.3, “MySQL Server and Server-Startup Programs”. The opposite of shutdown.

Answer 603

A form of replication where SQL statements are sent from the master server and replayed on the slave server. It requires some care with the setting for the innodb\_autoinc\_lock\_mode option, to avoid potential timing problems with auto-increment locking.

Answer 604

A form of replication where SQL statements are sent from the master server and replayed on the slave server. It requires some care with the setting for the innodb\_autoinc\_lock\_mode option, to avoid potential timing problems with auto-increment locking.

Answer 605

A form of replication where SQL statements are sent from the master server and replayed on the slave server. It requires some care with the setting for the innodb\_autoinc\_lock\_mode option, to avoid potential timing problems with auto-increment locking.

Answer 606

Estimated values relating to each InnoDB table and index, used to construct an efficient query execution plan. The main values are the cardinality (number of distinct values) and the total number of table rows or index entries. The statistics for the table represent the data in its primary key index. The statistics for a secondary index represent the rows covered by that index. The values are estimated rather than counted precisely because at any moment, different transactions can be inserting and deleting rows from the same table. To keep the values from being recalculated frequently, you can enable persistent statistics, where the values are stored in InnoDB system tables, and refreshed only when you issue an ANALYZE TABLE statement. You can control how NULL values are treated when calculating statistics through the innodb\_stats\_method configuration option. Other types of statistics are available for database objects and database activity through the INFORMATION\_SCHEMA and PERFORMANCE\_SCHEMA tables.

Answer 607

Estimated values relating to each InnoDB table and index, used to construct an efficient query execution plan. The main values are the cardinality (number of distinct values) and the total number of table rows or index entries. The statistics for the table represent the data in its primary key index. The statistics for a secondary index represent the rows covered by that index. The values are estimated rather than counted precisely because at any moment, different transactions can be inserting and deleting rows from the same table. To keep the values from being recalculated frequently, you can enable persistent statistics, where the values are stored in InnoDB system tables, and refreshed only when you issue an ANALYZE TABLE statement. You can control how NULL values are treated when calculating statistics through the innodb\_stats\_method configuration option. Other types of statistics are available for database objects and database activity through the INFORMATION\_SCHEMA and PERFORMANCE\_SCHEMA tables.

Answer 608

Estimated values relating to each InnoDB table and index, used to construct an efficient query execution plan. The main values are the cardinality (number of distinct values) and the total number of table rows or index entries. The statistics for the table represent the data in its primary key index. The statistics for a secondary index represent the rows covered by that index. The values are estimated rather than counted precisely because at any moment, different transactions can be inserting and deleting rows from the same table. To keep the values from being recalculated frequently, you can enable persistent statistics, where the values are stored in InnoDB system tables, and refreshed only when you issue an ANALYZE TABLE statement. You can control how NULL values are treated when calculating statistics through the innodb\_stats\_method configuration option. Other types of statistics are available for database objects and database activity through the INFORMATION\_SCHEMA and PERFORMANCE\_SCHEMA tables.

Answer 609

The ability to search for different variations of a word based on a common root word, such as singular and plural, or past, present, and future verb tense. This feature is currently supported in MyISAM full-text search feature but not in FULLTEXT indexes for InnoDB tables.

Answer 610

The ability to search for different variations of a word based on a common root word, such as singular and plural, or past, present, and future verb tense. This feature is currently supported in MyISAM full-text search feature but not in FULLTEXT indexes for InnoDB tables.

Answer 611

The ability to search for different variations of a word based on a common root word, such as singular and plural, or past, present, and future verb tense. This feature is currently supported in MyISAM full-text search feature but not in FULLTEXT indexes for InnoDB tables.

Answer 612

In a FULLTEXT index, a word that is considered common or trivial enough that it is omitted from the search index and ignored in search queries. Different configuration settings control stopword processing for InnoDB and MyISAM tables. See Section 12.9.4, “Full-Text Stopwords” for details.

Answer 613

In a FULLTEXT index, a word that is considered common or trivial enough that it is omitted from the search index and ignored in search queries. Different configuration settings control stopword processing for InnoDB and MyISAM tables. See Section 12.9.4, “Full-Text Stopwords” for details.

Answer 614

In a FULLTEXT index, a word that is considered common or trivial enough that it is omitted from the search index and ignored in search queries. Different configuration settings control stopword processing for InnoDB and MyISAM tables. See Section 12.9.4, “Full-Text Stopwords” for details.

Answer 615

A component of the MySQL database that performs the low-level work of storing, updating, and querying data. In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables, superceding MyISAM. Different storage engines are designed with different tradeoffs between factors such as memory usage versus disk usage, read speed versus write speed, and speed versus robustness. Each storage engine manages specific tables, so we refer to InnoDB tables, MyISAM tables, and so on. The MySQL Enterprise Backup product is optimized for backing up InnoDB tables. It can also back up tables handled by MyISAM and other storage engines.

Answer 616

A component of the MySQL database that performs the low-level work of storing, updating, and querying data. In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables, superceding MyISAM. Different storage engines are designed with different tradeoffs between factors such as memory usage versus disk usage, read speed versus write speed, and speed versus robustness. Each storage engine manages specific tables, so we refer to InnoDB tables, MyISAM tables, and so on. The MySQL Enterprise Backup product is optimized for backing up InnoDB tables. It can also back up tables handled by MyISAM and other storage engines.

Answer 617

A component of the MySQL database that performs the low-level work of storing, updating, and querying data. In MySQL 5.5 and higher, InnoDB is the default storage engine for new tables, superceding MyISAM. Different storage engines are designed with different tradeoffs between factors such as memory usage versus disk usage, read speed versus write speed, and speed versus robustness. Each storage engine manages specific tables, so we refer to InnoDB tables, MyISAM tables, and so on. The MySQL Enterprise Backup product is optimized for backing up InnoDB tables. It can also back up tables handled by MyISAM and other storage engines.

Answer 618

The general name for the setting controlled by the innodb\_strict\_mode option. Turning on this setting causes certain conditions that are normally treated as warnings, to be considered errors. For example, certain invalid combinations of options related to file format and row format, that normally produce a warning and continue with default values, now cause the CREATE TABLE operation to fail. MySQL also has something called strict mode.

Answer 619

The general name for the setting controlled by the innodb\_strict\_mode option. Turning on this setting causes certain conditions that are normally treated as warnings, to be considered errors. For example, certain invalid combinations of options related to file format and row format, that normally produce a warning and continue with default values, now cause the CREATE TABLE operation to fail. MySQL also has something called strict mode.

Answer 620

The general name for the setting controlled by the innodb\_strict\_mode option. Turning on this setting causes certain conditions that are normally treated as warnings, to be considered errors. For example, certain invalid combinations of options related to file format and row format, that normally produce a warning and continue with default values, now cause the CREATE TABLE operation to fail. MySQL also has something called strict mode.

Answer 621

Within the list structure that represents the buffer pool, pages that are relatively old and relatively new are represented by different portions of the list. A set of parameters control the size of these portions and the dividing point between the new and old pages.

Answer 622

Within the list structure that represents the buffer pool, pages that are relatively old and relatively new are represented by different portions of the list. A set of parameters control the size of these portions and the dividing point between the new and old pages.

Answer 623

Within the list structure that represents the buffer pool, pages that are relatively old and relatively new are represented by different portions of the list. A set of parameters control the size of these portions and the dividing point between the new and old pages.

Answer 624

A pseudo-record in an index, representing the gap above the largest value in that index. If a transaction has a statement such as SELECT ... FOR UPDATE ... WHERE col \> 10;, and the largest value in the column is 20, it is a lock on the supremum record that prevents other transactions from inserting even larger values such as 50, 100, and so on.

Answer 625

A pseudo-record in an index, representing the gap above the largest value in that index. If a transaction has a statement such as SELECT ... FOR UPDATE ... WHERE col \> 10;, and the largest value in the column is 20, it is a lock on the supremum record that prevents other transactions from inserting even larger values such as 50, 100, and so on.

Answer 626

A pseudo-record in an index, representing the gap above the largest value in that index. If a transaction has a statement such as SELECT ... FOR UPDATE ... WHERE col \> 10;, and the largest value in the column is 20, it is a lock on the supremum record that prevents other transactions from inserting even larger values such as 50, 100, and so on.

Answer 627

Synonym name for synthetic key.

Answer 628

Synonym name for synthetic key.

Answer 629

Synonym name for synthetic key.

Answer 630

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