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11 Cards in this Set
- Front
- Back
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Sequential file |
A file stored in key field order. No direct access, need to start reading records from the start of file |
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Indexed sequential file |
A sequential file that has an index that contains the key and the location. Can be accessed directly or sequentially using the index. |
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Multi level index |
On a very large indexed sequential file the index may be too large so it too can be indexed. This index if too large can also be indexed and so on. To access a record search the top level of index this indicates the next index to search serially. the final index will give the position of the file. |
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Direct (random) |
A file where the key is the position of the record in the file. If the key is known you can go directly to the record. New records are normally added at the bottom. |
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Hashed file |
A direct access file. A calculation is done on the key to give the position in the file. The position may not be unique so if it is occupied the record can be placed in OVERFLOW AREA. Searching: Hash the key to give the record position. If the record is not there then search the overflow area serially.
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Hash function |
The calculation applied to a key to give the position in a file. |
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Reorganisation |
If records are in an overflow area they take longer to find. If there have been many records put into an overflow area it may be worth reorganising the file. Records are copied from the old file into a new file possibly using the new hash function |
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File security |
Security means making sure data is not lost through hardware/software or user area. Backups can be taken regularly. Each backup is called a generation |
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Transaction logs |
A transaction log records changes made since the last backup. If a file is damaged it can be recreated from the backup and transaction log. |
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File privacy |
File privacy means making sure only the correct users are able to access a file. This can be achieved through ACCESS RIGHTS. A user may have full,read,write,read/write or no access. A password may be used to identify the user. |
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File encryption |
Scrambles the data in a file using a key. Only a user who has the key and the decryption algorithm can unscramble the data and access the file. |
