Chapter Seventeen

Access method An operating system algorithm for storing and locating data in secondary memory. See also File organization.
Audit trail A list of changes to a data file which allows business transactions to be traced. Both the updating and use of data should be recorded in the audit trail, since the consequences of bad data should be discovered and corrected.
Backward recovery (rollback) An approach to rebuilding a file in which before images of changed records are restored to the file in reverse order until some earlier state is achieved. See also Forward recovery (rollforward).
Blocking factor The number of physical records per page.
Calculated field A field which can be derived from other database fields. Also called computed or derived field.
Data compression technique Pattern matching and other methods which replace repeating strings of characters with codes of shorter length.
Data type A detailed coding scheme recognized by system software for representing organizational data.
Database management system (DBMS) Software that is used to create, maintain, and provide controlled access to user databases.
Default value A value a field will assume unless an explicit value is entered for that field.
Encryption The coding (or scrambling) of data so that they cannot be read by humans.
Field The smallest unit of named application data recognized by system software.
File organization A technique for physically arranging the records of a file on secondary storage devices. See also Access method, Hashed file organization, Indexed file organization, Sequential file organization.
Forward recovery (rollforward) An approach to rebuilding a file in which one starts with an earlier version of the file and either reruns prior transactions or replaces a record with its image after each transaction. See also Backward recovery (rollback).
Hashed file organization The address for each record is determined using a hashing algorithm.
Hashing algorithm A routine that converts a primary key value into a relative record number (or relative file address).
Index A table or other data structure used to determine the location of rows in a file that satisfy some condition.
Indexed file organization The records are either stored sequentially or nonsequentially and an index is created that allows software to locate individual records.
Null value A special field value, distinct from 0, blank, or any other value, that indicates that the value for the field is missing or otherwise unknown.
Page The amount of data read or written in one secondary memory (disk) input or output operation. For I/O with a magnetic tape, the equivalent term is record block. See also Blocking factor.
Physical file A named set of contiguous records.
Physical record A group of fields stored in adjacent memory locations and retrieved together as a unit.
Picture (or template) A pattern of codes that restricts the width and possible values for each position of a field.
Pointer A field of data that can be used to locate a related field or record of data.
Record partitioning The process of splitting logical records into separate physical segments based on affinity of use. See also Horizontal partitioning, Vertical partitioning.
Secondary key One or a combination of fields for which more than one record may have the same combination of values.
Sequential file organization The records in the file are stored in sequence according to a primary key value.

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