Tailoring for Performance › Using Select/Omit Maintenance

Using Select/Omit Maintenance

Select/omit maintenance filters records that match the specified selection or omission maintenance. There are two types of select/omit maintenance: static and dynamic.

If the select/omit maintenance is dynamic, all records, regardless of the select/omit set, are included in the access path. The records are filtered by the system as they are read by a program. Only those that match the select/omit maintenance are actually returned to the program.

If the select/omit criteria is static, only those records that satisfy the select/omit maintenance are included in the access path. As each record is added or changed, the system determines if it should be included in the access path. As the data is read, no filtering is required since it has already been performed by the access path maintenance.

Because of this difference, any access path with static select/omit criteria usually has a separate internal active index, whereas any access path with dynamic select/omit maintenance can share an active index with other similarly keyed access paths even if the select/omit criteria differ. The provision and use of dynamic select/omits increases the possibilities of sharing active indexes.

Note: For join logical files in CA 2E, (files with virtual fields) with select/omit criteria, static select/omit maintenance cannot be used. Use dynamic select/omit maintenance.

The type of data file is again important. For non-volatile (master or table) data files the logical files should, if possible, have static select/omit criteria. Static select/omit maintenance does require that an extra active index be maintained. But the frequency of change to the data is low and therefore maintenance does not occur often. For volatile (transaction) data files, the type of select/omit criteria depends on the actual number of records being read and the frequency of use of the active index.

If the active index is used infrequently or in batch, the overhead of having dynamic select/omit criteria may be acceptable.