Date: Wed, 28 Nov 2007 00:38:33 +0100
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
To: cluster <skrald@amossen.dk>
Cc: pgsql-performance@postgresql.org
Subject: Re: Query only slow on first run
Message-ID: <20071127233833.GA24031@uio.no>
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On Tue, Nov 27, 2007 at 11:51:40PM +0100, cluster wrote:
> A thing that strikes me is the following. As you can see I have the 
> constraint: q.status = 1. Only a small subset of the data set has this 
> status. I have an index on q.status but for some reason this is not used. 
> Instead the constraint are ensured with a "Filter: (q.status = 1)" in an 
> index scan for the primary key in the "q" table. If the small subset having 
> q.status = 1 could be isolated quickly using an index, I would expect the 
> query to perform better. I just don't know why the planner doesn't use the 
> index on q.status.

An index scan (as opposed to a bitmap index scan) can only use one index at a
time, so it will choose the most selective one. Here it quite correctly
recognizes that there will only be one matching record for the given
question_id, so it uses the primary key instead.

You could make an index on (question_id,status) (or a partial index on
question id, with status=1 as the filter), but I'm not sure how much it would
help you unless the questions table is extremely big. It doesn't appear to
be; in fact, it appears to be all in RAM, so that's not your bottleneck.

/* Steinar */
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Homepage: http://www.sesse.net/