I've seen a couple cases recently where CMOD has performed much worse after no change to CMOD, but an upgrade to DB2 V9.
This is a quote:
Quoteit appears that running
runstats on our index data tables solved the problem. Even though there
were no deletes and no static binds, RUNSTATS was still necessary for
these tables to account for change in cardinality. ....
... Presumably this
was noticed when we migrated to version 9 because the optimization
algorithms required greater accuracy of the catalog statistics.
Well well well --- and I just found this today:
DB2 9 migration: SQL performance best practices
http://www-01.ibm.com/support/docview.wss?rs=64&context=SSEPEK&q1=Db2+9+migration&uid=swg27015988
Well worth the read!