ch17-updated-2019-03-08

Question 1

What is a transaction?

Answer 1

A unit of program execution that accesses and possibly updates various data items.

Question 2

What is the atomicity requirement in transactions?

Answer 2

Either all operations of the transaction are properly reflected in the database or none are.

Question 3

What does the durability requirement state?

Answer 3

Once a transaction has completed, the changes must persist even if there are system failures.

Question 4

Define the consistency requirement in transactions.

Answer 4

A transaction must see a consistent database, and execution must preserve the consistency of the database.

Question 5

What is the isolation requirement?

Answer 5

Each transaction must be unaware of other concurrently executing transactions.

Question 6

What are the ACID properties of transactions?

Answer 6

• Atomicity
• Consistency
• Isolation
• Durability

Question 7

What are the possible states of a transaction?

Answer 7

• Active
• Partially committed
• Failed
• Aborted
• Committed

Question 8

What does a schedule in transaction processing refer to?

Answer 8

A sequence of instructions that specify the chronological order in which instructions of concurrent transactions are executed.

Question 9

What is conflict serializability?

Answer 9

A schedule is conflict serializable if it can be transformed into a serial schedule by swapping non-conflicting instructions.

Question 10

What is view serializability?

Answer 10

A schedule is view serializable if it is view equivalent to a serial schedule.

Question 11

What is the significance of a precedence graph in testing for conflict serializability?

Answer 11

A schedule is conflict serializable if and only if its precedence graph is acyclic.

Question 12

What is a recoverable schedule?

Answer 12

If a transaction reads a data item previously written by another transaction, the commit operation of the first must appear before the commit operation of the second.

Question 13

What is a cascading rollback?

Answer 13

A single transaction failure leads to a series of transaction rollbacks.

Question 14

Define cascadeless schedules.

Answer 14

Schedules where cascading rollbacks cannot occur; the commit operation of a transaction appears before any transaction that reads its data.

Question 15

What is the goal of concurrency control in databases?

Answer 15

To ensure that all possible schedules are either conflict or view serializable, and are recoverable and preferably cascadeless.

Question 16

Fill in the blank: A transaction that cannot proceed due to a logical error is in the _______ state.

Answer 16

Failed

Question 17

True or False: All conflict serializable schedules are also view serializable.

Answer 17

True

Question 18

What happens if a transaction fails after step 3 and before step 6 in a fund transfer?

Answer 18

Money will be ‘lost’, leading to an inconsistent database state.

Question 19

What is the benefit of executing multiple transactions concurrently?

Answer 19

Increased processor and disk utilization, leading to better transaction throughput.

Question 20

What does the term ‘conflicting instructions’ refer to?

Answer 20

Instructions that access the same data item and at least one of which writes to that data item.

Question 21

What is the main issue with erroneous transaction logic?

Answer 21

It can lead to an inconsistent database state.

Question 22

What is the significance of ensuring isolation in transaction processing?

Answer 22

It prevents other transactions from accessing a partially updated database, maintaining consistency.

Question 23

What algorithmic complexity is associated with cycle-detection in precedence graphs?

Answer 23

Order n^2 time, with better algorithms taking order n + e.

Question 24

Fill in the blank: A transaction that has successfully completed is in the _______ state.

Answer 24

Committed

Question 25

What must be ensured about the final write operation in view equivalence?

Answer 25

The same transaction must perform the final write operation in both schedules.

Question 26

What is the goal of developing concurrency control protocols?

Answer 26

To assure serializability ## Footnote Serializability ensures that transactions are executed in a way that the end result is the same as if they had been executed in some serial order.

Question 27

What are the necessary characteristics of schedules for database consistency?

Answer 27

Must be conflict or view serializable, and recoverable, preferably cascadeless ## Footnote Cascadeless schedules prevent cascading rollbacks, which can lead to lost updates.

Question 28

What is a policy that generates serial schedules?

Answer 28

Only one transaction can execute at a time ## Footnote This policy provides a poor degree of concurrency.

Question 29

What do concurrency-control schemes trade off?

Answer 29

The amount of concurrency they allow and the amount of overhead they incur.

Question 30

What types of schedules can some concurrency control schemes generate?

Answer 30

Conflict-serializable schedules and view-serializable schedules that are not conflict-serializable.

Question 31

What do concurrency-control protocols ensure about schedules?

Answer 31

They are conflict/view serializable, recoverable, and cascadeless.

Question 32

Do concurrency control protocols examine the precedence graph during creation?

Answer 32

No, they impose a discipline that avoids non-serializable schedules.

Question 33

What is the purpose of tests for serializability?

Answer 33

To help understand why a concurrency control protocol is correct.

Question 34

What is an example of a weak level of consistency?

Answer 34

Read-only transaction that wants an approximate total balance of all accounts.

Question 35

What can be traded off for performance in weak consistency applications?

Answer 35

Accuracy.

Question 36

What is the default level of consistency in SQL-92?

Answer 36

Serializable.

Question 37

What does Repeatable Read level of consistency ensure?

Answer 37

Only committed records to be read; repeated reads of the same record must return the same value.

Question 38

What allows successive reads of a record to return different values?

Answer 38

Read committed level of consistency.

Question 39

What does Read Uncommitted allow?

Answer 39

Even uncommitted records may be read.

Question 40

What do lower degrees of consistency allow?

Answer 40

Gathering approximate information about the database.

Question 41

What is a warning regarding some database systems?

Answer 41

They do not ensure serializable schedules by default.

Question 42

How does a transaction begin in SQL?

Answer 42

Implicitly.

Question 43

What are the two ways a transaction in SQL can end?

Answer 43

* Commit work * Rollback work

Question 44

What happens by default in almost all database systems when a SQL statement executes successfully?

Answer 44

Every SQL statement commits implicitly.

Question 45

How can implicit commit be turned off?

Answer 45

By a database directive.

Question 46

What can be set at the database level regarding transactions?

Answer 46

Isolation level.

Question 47

What is an example of setting the isolation level in SQL?

Answer 47

set transaction isolation level serializable.

Question 48

What is a method of implementing isolation levels?

Answer 48

Locking.

Question 49

What are the two types of locks mentioned?

Answer 49

* Shared locks * Exclusive locks

Question 50

What do timestamps help detect?

Answer 50

Out of order accesses.

Question 51

What do multiple versions of each data item allow?

Answer 51

Transactions to read from a 'snapshot' of the database.

Question 52

What is an example of a transaction in SQL?

Answer 52

select ID, name from instructor where salary > 90000.

Question 53

What does tuple level locking detect?

Answer 53

Conflicts.

Question 54

What phenomenon is illustrated by the example involving T1 and T2?

Answer 54

The phantom phenomenon.

Question 55

What is key to detecting 'predicate' conflicts?

Answer 55

Use some form of 'predicate locking'.