From: Tom Lane <tgl@sss.pgh.pa.us>
To: Neil Whelchel <neil.whelchel@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Slow count(*) again...
Date: Wed, 13 Oct 2010 09:49:12 -0400
Message-ID: <24768.1286977752@sss.pgh.pa.us> (raw)
In-Reply-To: <201010122347.20542.neil.whelchel@gmail.com>
References: <201010091626.18636.neil.whelchel@gmail.com>
<4CB2410C.6010603@postnewspapers.com.au>
<201010102115.58729.neil.whelchel@gmail.com>
<201010122347.20542.neil.whelchel@gmail.com>
Neil Whelchel <neil.whelchel@gmail.com> writes:
> Insert the data into one table:
> crash:~# time psql -U test test -q < log.sql
> real 679m43.678s
> user 1m4.948s
> sys 13m1.893s
> crash:~# echo 3 > /proc/sys/vm/drop_caches
> crash:~# time psql -U test test -c "SELECT count(*) FROM log;"
> count
> ----------
> 10050886
> (1 row)
> real 0m11.812s
> user 0m0.000s
> sys 0m0.004s
> crash:~# time psql -U test test -c "SELECT count(*) FROM log;"
> count
> ----------
> 10050886
> (1 row)
> real 0m3.737s
> user 0m0.000s
> sys 0m0.000s
> As can be seen here, the cache helps..
That's probably got little to do with caching and everything to do with
setting hint bits on the first SELECT pass.
I concur with Mark's question about whether your UPDATE pushed the table
size across the limit of what would fit in RAM.
regards, tom lane
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Cc: tgl@sss.pgh.pa.us, neil.whelchel@gmail.com
Subject: Re: Slow count(*) again...
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