Subject: Re: [Fwd: Re: Outer joins and Seq scans]
From: Sami Dalouche <skoobi@free.fr>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
In-Reply-To: <4121.1193923796@sss.pgh.pa.us>
References: <1193865739.23774.2.camel@samlaptop>
	<1193910660.29867.3.camel@samlaptop>  <4121.1193923796@sss.pgh.pa.us>
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Date: Thu, 01 Nov 2007 15:21:17 +0100
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Thanks for your answer.

So, basically, what you are saying is that there is nothing particularly
wrong with the query, nor with its optimization ? So if I need
performance for this query, I should just revert to other techniques
(giving more memory to postgres, caching outside postgres, etc..) ?

Regards,
Sami Dalouche

Le jeudi 01 novembre 2007 à 09:29 -0400, Tom Lane a écrit :
> Sami Dalouche <skoobi@free.fr> writes:
> > Compare that to the following query, that is exactly the same except
> > that the City table is inner'joined instead of outer joined
> > ...
> > the explain analyze is available at :
> > http://www.photosdesami.com/temp/exp6.txt
> 
> AFAICS it's just absolutely blind luck that that query is fast.  The
> planner chooses to do the contactinf7_/city8_ join first, and because
> that happens to return no rows at all, all the rest of the query falls
> out in no time, even managing to avoid the scan of adcreatedevent.
> If there were any rows out of that join it would be a great deal slower.
> 
> There is a pretty significant semantic difference between the two
> queries, too, now that I look closer: when you make 
> "... join City city8_ on contactinf7_.city_id=city8_.id"
> a plain join instead of left join, that means the join to contactinf7_
> can be reduced to a plain join as well, because no rows with nulls for
> contactinf7_ could possibly contribute to the upper join's result.
> That optimization doesn't apply in the original form of the query,
> which restricts the planner's freedom to rearrange things.
> 
> 			regards, tom lane