Determining How an Entity Should Be Stored › Location Modes › Randomization

Randomization

CALC location mode

CA IDMS/DB allows users to distribute occurrences of a particular entity randomly across the area to which it is assigned. Randomization of entity occurrences is achieved with the CALC location mode.

When you specify CALC for an entity, the database uses a randomizing algorithm to calculate a storage page for each occurrence of that entity; the calculation is based on the value of a symbolic key (called the CALC key).

The diagram below shows the use of the CALC location mode to randomize entity occurrences.

CA IDMS/DB stores an occurrence of a CALC entity on or near a calculated storage page. The entity is placed directly on the preferred page if sufficient space exists. Otherwise, it is placed on the next page within the area where sufficient space exists. If the end of the area is reached in the search for space, CA IDMS/DB wraps around to the beginning of the area.

Purpose of the CALC location mode

The purpose of the CALC location mode is twofold:

• Direct retrieval by symbolic key, enabling retrieval of an entity occurrence with a single read operation. Retrieval of an entity located CALC involves knowing only the value of its CALC key; the database automatically converts the CALC key into the correct page number when the entity is requested. For more information concerning the use of numeric fields within a record's CALC key, see Zoned and Packed Decimal Fields as IDMS Keys.
• Random distribution of entity occurrences over all the pages in an area. This reduces overflow conditions and leaves space for clustered entity occurrences. For further information on overflow conditions, see "Overflow Conditions" in Chapter 15, "Determining the Size of the Database".