DL/I and CA IDMS/DB › Secondary Indexing (Index Databases)

Secondary Indexing (Index Databases)

What is a Secondary Index

A secondary index defines an alternative (or secondary) access path that overrides the underlying hierarchical access path. DL/I supports the following types of secondary indexes:

• An index to a root or dependent segment on the basis of any field in the segment
• An index to a root or dependent segment on the basis of any field in a physically dependent segment

The key field for an index can be a single field or up to five fields in the same segment concatenated in any order. A physical database can have multiple secondary indexes.

Define Secondary Index as a Separate Database

Secondary indexes must be defined as separate databases. The segment occurrences in a secondary index database contain the values of the specified key field(s) and the pointers to the associated segment occurrences in the physical database. The secondary index segment is known as the pointer segment. The segment containing the key field(s) is known as the source segment and the segment to be accessed is known as the target segment. The source and target segments can be the same or different.

Secondary indexes differ from HIDAM indexes in that they allow you to index segments other than root segments. In a secondary index, the pointer segment can contain up to five concatenated fields, rather than just one field. Also, the source and target segments do not have to be the same.