From: Tom Lane <tgl@sss.pgh.pa.us>
To: Marcio Ribeiro <mribeiro@gmail.com>
cc: pgsql-performance@postgresql.org
Subject: Re: LIMIT 1 poor query plan
In-reply-to: 
 <CAMAHXRzu6daCPNtOGcGboTAsQ5gs9vdFEsTvACC2b6if9ni2-w@mail.gmail.com>
References: 
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Comments: In-reply-to Marcio Ribeiro <mribeiro@gmail.com>
 message dated "Sat, 10 Oct 2015 05:52:35 -0300"
Date: Sat, 10 Oct 2015 10:45:36 -0500
Message-ID: <48410.1444491936@sss.pgh.pa.us>
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Sender: pgsql-performance-owner@postgresql.org

Marcio Ribeiro <mribeiro@gmail.com> writes:
> Short question:
> Why would pg optimizer choose a worst (slower) query plan for a
> query with 'LIMIT 1' instead of, say, 'LIMIT 3'?

> Complete scenario:
> Query: 'SELECT * FROM a WHERE a.b_id = 42 ORDER BY created LIMIT 1'
> - b_id is a FK to b;
> - created is a datetime with the time of the creation of the row;
> - both 'b' and 'created' are indexed separately

> This query, with the LIMIT 1, uses the index on created, which is much
> slower (10x) than if it used the index on b_id

It's trying to avoid a sort; or to be less anthropomorphic, the estimated
cost of scanning the "created" index until it hits the first row with
b_id=42 is less than the estimated cost of collecting all the rows with
b_id=42 and then sorting them by "created".  The estimates unfortunately
are kind of shaky because it's hard to predict how many rows will get
skipped before finding one with b_id=42.

If you do this type of query often enough to care about its performance,
you could consider creating a two-column index on (b_id, created)
(in that order).

			regards, tom lane


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