Subject: Re: Outer joins and equivalence
From: Simon Riggs <simon@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
In-Reply-To: <14085.1211924592@sss.pgh.pa.us>
References: <1211918358.4489.257.camel@ebony.site>
	<14085.1211924592@sss.pgh.pa.us>
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Date: Mon, 02 Jun 2008 18:10:44 +0100
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On Tue, 2008-05-27 at 17:43 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndquadrant.com> writes:
> > I have a complex query where making a small change to the SQL increases
> > run-time by > 1000 times.
> 
> > The first SQL statement is of the form
> 
> > A JOIN B ON (a.id = b.id) LEFT JOIN C ON (a.id = c.id) 
> 
> > and the second is like this
> 
> > A JOIN B ON (a.id = b.id) LEFT JOIN C ON (b.id = c.id)
> 
> > the only difference is the substitution of a -> b
> 
> Please provide an actual test case.

Getting closer, but still not able to produce a moveable test case.

Symptoms are

* using partitioning
* when none of the partitions are excluded
* when equivalence classes ought to be able to reconcile join 

Still working on it

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support