Primarily sequential processing: HISAM

HISAM is an access method that stores segments in hierarchic sequence with an index to locate root segments. It also has an overflow data set. Store segments in a logical record until you reach the end of the logical record. When you run out of space on the logical record, but you still have more segments belonging to the database record, you store the remaining segments in an overflow data set. The access methods that HISAM can use are VSAM and OSAM.

HISAM is well-suited for:

• Direct access of record by root keys
• Sequential access of records
• Sequential access of dependent segments

The situations in which your processing has some of these characteristics but where HISAM is not necessarily a good choice, occur when:

• You must access dependents directly.
• You have a high number of inserts and deletes.
• Many of the database records exceed average size and must use the overflow data set. The segments that overflow into the overflow data set require additional I/O.

HISAM characteristics

For database records, HISAM databases:

• Store records in key sequence
• Can locate a particular record with a key value by using the index

For dependent segments, HISAM databases:

• Start each HISAM database record in a new logical record in the primary data set
• Store the remaining segments in one or more logical records in the overflow data set if the database record does not fit in the primary data set

An overview of how HISAM works

This topic contains Diagnosis, Modification, and Tuning information.

HISAM does not immediately reuse space. When you insert a new segment, HISAM databases shift data to make room for the new segment, and this leaves unused space after deletions. HISAM space is reclaimed when you reorganize a HISAM database.