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Re: Appending "LIMIT" to query drastically decreases performance

From: Bill Moran <wmoran(at)collaborativefusion(dot)com>
To: Matthew <matthew(at)flymine(dot)org>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Appending "LIMIT" to query drastically decreases performance
Date: 2007-11-30 14:20:33
Message-ID: 20071130092033.f09c1ac0.wmoran@collaborativefusion.com (view raw or flat)
Thread:
Lists: pgsql-performance
In response to Matthew <matthew(at)flymine(dot)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(at)collaborativefusion(dot)com
Phone: 412-422-3463x4023

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