Make sure you capture your top 20 sql during the peak production hour in 8i. Analyze and test it in 9i server first.
Oralce 9i optimizer is very sensitive and you need to have proper statistics collected on all tables and index to get an optimized performance.
If in case your performance degrades after upgrading to 9i, use the OPTIMIZER_FEATURES_ENABLE parameter to make oracle optimizer behave like the old version.
Good luck.
Senthil kumar
