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From: Tim Jones <TJones@optio.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: strange query behavior
Date: Thu, 14 Dec 2006 14:18:55 -0500
Message-ID: <47668A1334CDBF46927C1A0DFEB223D39F7C5D@mail.optiosoftware.com> (raw)
In-Reply-To: <22464.1166118515@sss.pgh.pa.us>
References: <47668A1334CDBF46927C1A0DFEB223D39F79A5@mail.optiosoftware.com>
	<10648.1166037392@sss.pgh.pa.us>
	<47668A1334CDBF46927C1A0DFEB223D39F79FA@mail.optiosoftware.com>
	<12988.1166052295@sss.pgh.pa.us>
	<47668A1334CDBF46927C1A0DFEB223D39F7B01@mail.optiosoftware.com>
	<22464.1166118515@sss.pgh.pa.us>

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@sss.pgh.pa.us] 
Sent: Thursday, December 14, 2006 12:49 PM
To: Tim Jones
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] strange query behavior 

"Tim Jones" <TJones@optio.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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