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Re: Outer joins and equivalence

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Outer joins and equivalence
Date: 2008-06-02 17:10:44
Message-ID: 1212426644.4120.378.camel@ebony.site (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
On Tue, 2008-05-27 at 17:43 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndquadrant(dot)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


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