To: PFC <lists@peufeu.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Interesting slow query 
In-reply-to: <op.ta1s0auacigqcu@apollo13> 
References: <003601c687ce$7aee7b20$7f00a8c0@kicommunication.com>
	<op.ta1s0auacigqcu@apollo13>
Comments: In-reply-to PFC <lists@peufeu.com>
	message dated "Mon, 12 Jun 2006 21:42:00 +0200"
Date: Mon, 12 Jun 2006 18:53:08 -0400
Message-ID: <13392.1150152788@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

PFC <lists@peufeu.com> writes:
> 	Here are two ways to phrase a query... the planner choses very different  
> plans as you will see. Everything is freshly ANALYZEd.

Usually we get complaints the other way around (that the NOT EXISTS
approach is a lot slower).  You did not show any statistics, but I
suspect the key point here is that the condition id > 1130306 excludes
most or all of the A and D tables.  The planner is not smart about
making transitive inequality deductions, but you could help it along
by adding the implied clauses yourself:

EXPLAIN ANALYZE SELECT r.* FROM raw_annonces r
  LEFT JOIN annonces a ON (a.id=r.id AND a.id > 1130306)
  LEFT JOIN archive_data d ON (d.id=r.id AND d.id > 1130306)
  WHERE a.id IS NULL AND d.id IS NULL AND r.id > 1130306
  order by id limit 1;

Whether this is worth doing in your app depends on how often you do
searches at the end of the ID range ...

			regards, tom lane