Michael_D. 1100004WAH Visits (13539)
PM56845: PROVIDE OPTION FOR OPTIMIZE FOR 1 ROW TO ALLOW SOME SORT ACCESS PATHhtt
In all versions of DB2, the OPTIMIZE FOR 1 ROW clause requests DB2 to choose an access path that avoids a sort. In DB2 versions prior to 10, there is a possibility to obtain an access path with a sort even though that path is strongly discouraged. In DB2 10, DB2 will not compete access paths with sorts and will instead choose the lowest cost access path that does not require a sort.
This APAR provides an option to return to the previous version OPTIMIZE FOR 1 ROW behavior. As such, it does not eliminate the risk of an inefficient access path being chosen with OPTIMIZE FOR 1 ROW when the efficient access requires a sort. However, it does limit that exposure to what already existed in DB2 prior to DB2 10.
For queries that need sorts, the recommended solution is to avoid coding the OPTIMIZE FOR 1 ROW clause. Without the OPTIMIZE FOR 1 ROW clause, DB2 will choose access paths based on cost and will not make an effort to avoid sorts.
Local work around:
For queries that need sorts for efficient access, the solution is to avoid coding the OPTIMIZE FOR 1 ROW clause.
Change application to code OPTIMIZE FOR 2 ROWS
Wait for APAR PM56845 that is now open to provide option for OPTIMIZE FOR 1 ROW to allow sort access plans