One of my client's database performance degraded after upgrading from 8.1.7 to 9.2.0. I am aksing them to send me the STATSPACK report so I am still waiting. I did some research and it may be very well having something to do with optimizer_features_enable parameter. B/c of this parameter, the impact could relate to many other parameter in 9.2.0 since it's so different between the two version.
One of the suggestion is that revert this parameter back to 8.1.7. My question is what can we do if we want to leave this parameter in 9.2.0.