Specifying a sparse index

First, you can code a value in the NULLVAL= operand on the XDFLD statement in the DBD that equals the condition under which you do not need a pointer segment put in the index. You can put BLANK, ZERO, or any 1-byte value (for example, X'10', C'Z', 5, or B'00101101') in the NULLVAL= operand.

• BLANK is the same as C ' ' or X'40'
• ZERO is the same as X'00' but not C'0'

When using the NULLVAL= operand, a pointer segment is suppressed if every byte of the source field has the value you used in the operand.

Second, if the values you are allowed to code in the NULLVAL= operand do not work for you, you can create an index maintenance exit routine that determines the condition under which you do not need a pointer segment put in the index. If you create your own index maintenance exit routine, you code its name in the EXTRTN= operand on the XDFLD statement in the DBD. You can only have one index maintenance exit routine for each secondary index. This exit routine, however, can be a general purpose one that is used by more than one secondary index.

The exit routine must be consistent in determining whether a particular pointer segment needs to be put in the index. The exit routine cannot examine the same pointer segment at two different times but only mark it for suppression once. Also, user data cannot be used by your exit routine to determine whether a pointer segment is to be put in the index. When a pointer segment needs to be inserted into the index, your exit routine only sees the actual pointer segment just before insertion. When a pointer segment is being replaced or deleted, only a prototype of the pointer segment is seen by your exit routine. The prototype contains the contents of the constant, search, subsequence, and duplicate data fields, plus the symbolic pointer if there is one.

The information needed to code a secondary index maintenance exit routine is in IMS Version 15.2 Exit Routines.