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10 Cards in this Set
- Front
- Back
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Sequenctial file |
A file stored in key field order. No direct access, starts reading records from start of file |
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Indexed sequential file |
A sequential file that has an index that contains a key and file position. Can be accessed sequentially or directly using the index |
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Multi level indexed |
On a very large indexed sequential file, the file index may be too large, so it too can be indexed. To access a record search the top level index, this links you to the next index. The final index will give the position of the file |
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Direct access file |
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 added at the end |
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Hashed file |
A direct access file. A calculation isndone 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 the overflow area. Searching: Hash the key to give the record position. If the record is not there, 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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Reorganization |
If records are in an overflow area they take longer to find. If there have. We. Many records put into overflow area it may be worth reorganizing the file. Records are copied from the old file to a new one, possibly using a new hash function |
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File security |
Security means making sure the 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 log |
A transaction log records changes made since the last backup If a file is damaged it can be recreated from the backup and the transaction log. |
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File privicy |
File privicy means making sure only the correct users are able to access a file. Access rights: a user may have full, read, write, read -write or no access. Encryption scrambles the data in a file using a key. Only a user who has the key and the decryption algorithm can unscrable the data and access the file. |
