Indices

Indices are a mechanism for accessing the rows of a catalogue as though they were sorted into ascending or descending order on some column. For example, if an ascending index was created on REAL column DEC and the rows of the catalogue were accessed through this index the rows would appear in ascending order of DEC. In CAT version 8.1 indices are temporary entities which persist only for the duration of the application which generated them and perish when it terminates. Future versions of CAT will support permanent indices which persist after the application which generated them terminates.

Indices can be created from columns of any of the numeric data types. They should not be created from columns of data type CHARACTER or LOGICAL. If an index is created on a column which contains null values then the rows for which the column is null will appear after all the rows with a valid value. The order of such rows is unpredictable.

The attributes of an index are listed in Table  and described below. They are all mandatory and immutable and are set automatically when the index is created.

Table: Attributes of an index

Attribute	Name	Data type
Column identifier	COLID	_INTEGER
Order	ORDER	_INTEGER
Number of rows	NUMSEL	_INTEGER
Comments	COMM	_CHAR
Modification date	DATE	_DOUBLE