When you issue a FOCUS report request, the adapter must determine how much of the request it can pass off to IMS (that is, translate into an IMS SSA). If IMS can apply the screening conditions and select the proper records from the database, FOCUS need only format the results. However, if the adapter cannot translate parts of the FOCUS request into an SSA, it must retrieve the segments sequentially from IMS and let FOCUS apply the screening criteria.

Optimization is the process by which the adapter passes the selection operations of a FOCUS request to IMS for processing. Adapter optimization reduces the volume of IMS-to-FOCUS communication and improves response time by exploiting IMS internal optimization techniques.

A qualified SSA (Segment Search Argument) for a segment is a logical condition on one or more sequence or search fields from the segment. IMS retrieves only the segment instances that make the condition true. For example, a qualified SSA on the STANINFO segment of the DI21PART database could be the following:

STANINFO(STANKEY EQ AA)

The adapter communicates with IMS by issuing GU (Get Unique) and GN (Get Next) DL/I calls. (See IMS Overview and Mapping Concepts if you are unfamiliar with the terms and concepts mentioned in this chapter.) The most efficient DL/I calls incorporate qualified SSAs and thus enable IMS to find the required segment instances. Therefore, when the adapter intercepts a FOCUS report request, its goal is to construct a set of qualified SSAs that completely describes the FOCUS screening criteria and to pass these SSAs to IMS.

Although you can issue any FOCUS report request through the adapter, keep the following in mind:

• Only fields that are IMS sequence fields, search fields, or secondary indexes (in the Master File, ALIAS=IMSname.KEY, .IMS, .HKY, or .SKY) can generate qualified SSAs. If the FOCUS request includes selection criteria on fields that are not sequence or search fields, the adapter retrieves the necessary segments sequentially, and FOCUS screens them.
• Screening conditions on fields in OCCURS segments cannot be optimized.
• Screening conditions on group fields can be optimized only if your request references either the group name or the left-most component fields (high-order fields) from the group. Secondary index fields are group fields and are subject to this same restriction.

See Record Selection Tests for a detailed discussion of screening conditions and optimization.