It sounds like that might be due to what Jonathan suggested and I also observed - that your queries aren't getting hard parsed in between executions so you're still seeing whatever was loaded in the shared pool prior to changing _gby_hash_aggretation_enabled. Try flushing one of the queries out of the pool (e.g. by granting permissions on one of the dependent tables), then rerun the query after changing the parameter and hopefully you'll see it work consistently. -----Original Message----- From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Taylor, Chris David Ok, now that's really weird. In one of my DBs it does in fact reverse, but in our main Psoft database it does not. Privileged/Confidential Information may be contained in this message or attachments hereto. Please advise immediately if you or your employer do not consent to Internet email for messages of this kind. Opinions, conclusions and other information in this message that do not relate to the official business of this company shall be understood as neither given nor endorsed by it. -- //www.freelists.org/webpage/oracle-l