Retrieving Data From RMS Files

By default, the primary key is used for retrieval of records from indexed RMS files. You can override this default with the Automatic Index Selection (AIS).

The primary benefit of keys is improved efficiency in record retrieval. They provide an alternate, more efficient, retrieval method and can be used with screening tests on the selected key which are translated into direct reads. Indexes can also be used to control the order of retrieval of records.

To screen more than one field when working with group keys that are comprised of multiple fields, use a slash (/) character to separate the components of the group key. Screening is limited to NE, IS, and EQ relationships. The syntax is

... groupname {NE|IS|EQ}[']value/value.../value[']

where:

groupname

Is the GROUP field name in the Master File.

The slash (/) separates the parts of the group field. If you omit the optional quotation marks, embedded blanks in the value will be removed. Use single quotations marks (') around the values to preserve the values exactly as typed.

The Automatic Index Selection (AIS) facility automatically selects a key for direct access to an indexed file based on a request.

AIS automatically uses a key for direct retrieval when an applicable screening condition is reached in a request. AIS is used when all of the following requirements are met:

• Primary keys are described in a Master File with the ALIAS=KEY or ALIAS=DKEY attribute placed on the GROUP declaration (not on every field in the GROUP). Note that the primary key must always be described as a GROUP.
• Secondary keys are described on the FIELD declaration for a secondary key not belonging to a group.
• There is an optimizable screening condition on a key field. If a key is defined as a GROUP, the screening condition must be on either the GROUP name, or on the first field of the key. Only those key fields in a screening condition with one of the following logical relations are used for index selection:
  • Equal
  • Less than or equal to
  • Less than
  • Greater than
  • Greater than or equal to
• AIS applies only to keys in the root segment.