There is not an optimization level setting on DB2 UDB for iSeries. The best way to find out which settings influence the optimizer is to attend the DB2 UDB for iSeries SQL Performance Workshop.
Portions of this workshop are online, just follow the links for Online DB2 Education here.
Optimizer hints are also not supported. Attending the workshop as well as reviewing the DB2 UDB for iSeries Index Strategy White Paper should give you a better understanding of how the iSeries optimizer processes indexes. If the optimizer is creating a temporary index that's an exact copy of the permanent index, then you may have an issue that should be reported to IBM Support.
================================== MORE INFORMATION ON THIS TOPIC ==================================
Search400.com's targeted search engine: Get relevant information on DB2/400.
The Best Web Links: tips, tutorials and more.
Check out this online event, Getting the Most out of SQL & DB2 UDB for the iSeries.
This was first published in February 2003