We support DB2 on both AIX and the iSeries. I believe on AIX (DB2) there is a way to set the optimizer sensitivity.
Is there a way on the iSeries to set DB2 optimizer parameters? Also, is there a way to give the optimizer hints? There are times where the optimizer chooses not to use an index (that already exists) but builds a temporary one instead. 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.
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