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Re: strange query behavior

From: "Tim Jones" <TJones(at)optio(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: strange query behavior
Date: 2006-12-14 19:18:55
Message-ID: 47668A1334CDBF46927C1A0DFEB223D39F7C5D@mail.optiosoftware.com (view raw or flat)
Thread:
Lists: pgsql-performance
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(at)sss(dot)pgh(dot)pa(dot)us] 
Sent: Thursday, December 14, 2006 12:49 PM
To: Tim Jones
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] strange query behavior 

"Tim Jones" <TJones(at)optio(dot)com> 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

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