From: Bill Moran <wmoran@collaborativefusion.com>
To: Matthew <matthew@flymine.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: Appending "LIMIT" to query drastically decreases performance
Date: Fri, 30 Nov 2007 09:20:33 -0500
Message-ID: <20071130092033.f09c1ac0.wmoran@collaborativefusion.com> (raw)
In-Reply-To: <Pine.LNX.4.58.0711301327520.3731@aragorn.flymine.org>
References: <fiordm$2aev$1@news.hub.org>
<Pine.LNX.4.58.0711301327520.3731@aragorn.flymine.org>
In response to Matthew <matthew@flymine.org>:
> On Fri, 30 Nov 2007, cluster wrote:
> > Can anyone explain the following odd behavior?
> > I have a query that completes in about 90 ms. If I append LIMIT to the
> > very end, eg. "LIMIT 500" the evaluation time increases to about 800 ms.
> > How can performance get *worse* by giving the database the option to
> > stop the evaluation earlier (when it reaches the output 500 rows)?
>
> The planner doesn't always get it right. Simple.
>
> Have you done a "VACUUM FULL ANALYSE" recently?
I don't know about the "FULL" part ... but certainly an ANALYZE.
Please post EXPLAIN ANALYZE output for the two queries.
--
Bill Moran
Collaborative Fusion Inc.
http://people.collaborativefusion.com/~wmoran/
wmoran@collaborativefusion.com
Phone: 412-422-3463x4023
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To: pgsql-performance@postgresql.org
Cc: wmoran@collaborativefusion.com, matthew@flymine.org
Subject: Re: Appending "LIMIT" to query drastically decreases performance
In-Reply-To: <20071130092033.f09c1ac0.wmoran@collaborativefusion.com>
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