After upgrading from 18.104.22.168 to 10.2.0.3, one of our systems suddenly slowed down. Not directly after the upgrade, but three days later. After some investigation, we found that some of the statistics had been estimated and not computed. That was pretty much a shock because my analyze method usually specifies estimate_percent=>null to make sure we compute. (Note, I already figured out that the default estimate_percent changed from 9i to 10g, so that wasn't it.)
I checked my custom analyze method and it reported that the table was never analyzed by me.
Hmmm, now it's getting real interesting. Just for kicks, I reran dbms_stats.gather_table_stats without an estimate_percent parameter and it estimated at about 8.3%. Aha! Must be using the AUTO_SAMPLE_SIZE for some reason.
Then I looked at the last_analyzed time of the tables that had been estimated. They all were run between 22:00 and 01:00. Aha, a schedule. Then I looked in dba_scheduler_jobs and there it was. A new job that Oracle automatically sets up to analyze stale objects.
When I checked in the morning, the objects didn't report as being stale. Then, after the day's data, Oracle decided the statistics were stale and analyzed the tables. When I went to gather statistics for stale objects the next morning, they had already been analyzed and weren't stale anymore. Except the statistics weren't optimal which made our queries "go bad".
Fortunately, there is a way you can disable this job just by tweaking the schedule.