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Re: Weird 8.2.4 performance

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Kurt Overberg <kurt(at)hotdogrecords(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Weird 8.2.4 performance
Date: 2007-06-07 13:32:31
Message-ID: 29877.1181223151@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Kurt Overberg <kurt(at)hotdogrecords(dot)com> writes:
> ... Turning off bitmap scans definitely seems  
> to help things,

I really seriously doubt that.  On queries like this, where each inner
scan is fetching a couple hundred rows, the small extra overhead of a
bitmap scan should easily pay for itself.  I think you're looking
entirely at caching effects that allow a re-read of the same data to
go faster.

You might try running the same query plan several times in a row and
noting the lowest time, then repeat for the other query plan.  This will
get you comparable fully-cached times, which I bet will be very close
to the same.

			regards, tom lane

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