File Organisation Flashcards
File
A file is collection of related records
Record
A record is a collection of related fields
Variable length field
Variable length record has different number of bytes in each record. Variable length field makes it difficult to calculate how much storage space will be required. Variable length field are slower to process by computer as start and end locations have to be calculated. Variable length field saves storage space as no blank space.
E.g. - Name, Address, email address.
Fixed length field
Fixed length field has the same number of bytes in each record. Fixed length field is easier to program as it can be calculated to know how much storage space will be required. Fixed length field are quicker to process by computer as start and end locations are known. Fixed length field wastes storage space as fields have blank space.
E.g. - Gender, Date Of Birth.
Serial File
Data is stored in the order in which it was entered. No order to the data is maintained. Useful for storing transactional and initialisation files. To add to a serial file the new record is appended to the end of the file.
Sequential Files
Data is stored in the order of a key filed. Order is maintained when new records are added. Useful for storing master files. To add to a sequential file, a new file is made by copying the old file until an insertion is required then inserting the new record and copying the rest of the file. The old file is then deleted.
Direct access
The physical location for the new record is calculated from the key field, a hashing algorithm is used for this calculation to find the location. Suitable for large files which need this sort of access.
Overflow
Overflow describes a situation where a disk block is full and a new record requires somewhere to be stored, the new record will be stored in this overflow area.
Indexed sequential
Records stored in order of a key field. An index allows data to be accessed directly, the key field and index are used to locate the position
