ok thanks Tom I will alter the statistics and re-analyze the table. Tim Jones Healthcare Project Manager Optio Software, Inc. (770) 576-3555 -----Original Message----- From: Tom Lane [mailto:tgl@xxxxxxxxxxxxx] Sent: Thursday, December 14, 2006 12:49 PM To: Tim Jones Cc: pgsql-performance@xxxxxxxxxxxxxx Subject: Re: [PERFORM] strange query behavior "Tim Jones" <TJones@xxxxxxxxx> writes: > 18,273,008 rows in observationresults > [ and n_distinct = 12942 ] OK, so the estimated rowcounts are coming from those two numbers. It's notoriously hard to get a decent n_distinct estimate from a small sample :-(, and I would imagine the number of batteryidentifiers is really a lot more than 12942? What you need to do is increase the statistics target for observationresults.batteryidentifier (see ALTER TABLE) and re-ANALYZE and see if you get a saner n_distinct in pg_stats. I'd try 100 and then 1000 as target. Or you could just increase the global default target (see postgresql.conf) but that might be overkill. It's still a bit odd that the case with two batteryidentifiers was estimated fairly accurately when the other wasn't; I'll go look into that. But in any case you need better stats if you want good plans. regards, tom lane
