From: Tom Lane <tgl@sss.pgh.pa.us>
To: Harry Hehl <Harry.Hehl@diskstream.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Query plan changing when queried data does not
Date: Mon, 18 Dec 2006 12:07:58 -0500
Message-ID: <10440.1166461678@sss.pgh.pa.us> (raw)
In-Reply-To: <6AD4F3A63B017C4FB074E2C895AD1854E31A60@EXCHSRV.waterloonetworking.net>
References: <6AD4F3A63B017C4FB074E2C895AD1854E31A60@EXCHSRV.waterloonetworking.net>
"Harry Hehl" <Harry.Hehl@diskstream.com> writes:
> Why does vacuum full and reindex make a difference if the 3 tables are
> never updated or records deleted?
Probably because you did an ANALYZE somewhere and updated the planner's
stats. I think your major problem is poor estimation of the ds_tables
result:
> -> Seq Scan on ds_tables (cost=0.00..59.80 rows=1
> width=48) (actual time=15.208..15.968 rows=121 loops=1)
> Filter: ((lower(name) <> 'ds_omdatatest'::text) AND
> (lower(name) <> 'ds_ommessage'::text) AND (lower(name) <>
> 'ds_omusersetting'::text) AND (lower(name) <> 'ds_omloginsession'::text)
> AND (lower(name) <> 'ds_omclassdef'::text) AND (lower(name) <>
> 'ds_omuser'::text) AND (lower(name) <> 'ds_omusergroupsetting'::text)
> AND (lower(name) <> 'ds_omtestobject'::text) AND (lower(name) <>
> 'ds_omhomedirectory'::text) AND (lower(name) ~~ 'ds_om%'::text))
If you have an index on lower(name) then ANALYZE will collect statistics
on it, and you'd get an estimate of the result size that was better than
random chance ... but I bet you have no such index. You might get some
improvement from raising the default statistics target, too.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, Harry.Hehl@diskstream.com
Subject: Re: Query plan changing when queried data does not
In-Reply-To: <10440.1166461678@sss.pgh.pa.us>
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