From: Tom Lane <tgl@sss.pgh.pa.us>
To: Bruno Baguette <bruno.baguette@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Slow SQL query (14-15 seconds)
Date: Thu, 13 Nov 2008 09:30:08 -0500
Message-ID: <14560.1226586608@sss.pgh.pa.us> (raw)
In-Reply-To: <491C377E.3070904@gmail.com>
References: <491C0962.9060405@gmail.com>
<5163.1226583110@sss.pgh.pa.us>
<491C377E.3070904@gmail.com>
Bruno Baguette <bruno.baguette@gmail.com> writes:
> Le 13/11/08 14:31, Tom Lane a écrit :
>> 16.511 * 818 = 13505.998, so this is all but about 100 msec of the
>> runtime. Can't tell if there's any easy way to improve it. In
>> pre-8.4 releases trying to convert the EXISTS into an IN might help.
> Can you explain why a IN is fastest than an EXISTS subquery ?
The planner is smarter about IN than EXISTS --- it can usually convert
the former into a join plan instead of a subplan. (This situation will
improve in 8.4.)
> Do you think I can improve again the performance of that query ?
You've still got a subplan in there, not quite sure why. Anyway,
increasing work_mem might get it to change to a hashed subplan,
which'd likely be faster.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, bruno.baguette@gmail.com
Subject: Re: Slow SQL query (14-15 seconds)
In-Reply-To: <14560.1226586608@sss.pgh.pa.us>
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