9i Statistics Bug & New Features
I posted this same message in another thread, but in case you didn't see it there, here it is:
There are documented bugs with dbms_stats in 9i, especially when computing stats on indexes. You may think it is computing stats, but if you check the stats manually you will see they are inaccurate. I would highly recommend that you check to see if you are experiencing this problem and if you are, then I recommend you use the old fashioned ANALYZE INDEX . . . COMPUTE STATISTICS for your indexes, OR apply the patch (3078144) from Oracle that fixes the problem. See Metalink note 270361.1 for more info.
A few other things: make sure you are using PGA_AGGREGATE_TARGET, LMT with ASSM, automatic UNDO and all the other new 9i features and have them tuned properly. If you continue to have performance problems, look at statspack and see where your bottleneck and what are the most poorly performing queries.
Hope this helps,