Subject: Re: Slow update with simple query
From: Ragnar <gnari@hive.is>
To: Arnaud Lesauvage <thewild@freesurf.fr>
Cc: Jens Schipkowski <jens.schipkowski@apus.co.at>,
	pgsql-performance@postgresql.org
In-Reply-To: <4580025D.2040903@freesurf.fr>
References: <457FDB1E.6040801@freesurf.fr>
	<op.tkhy4jpg81rjf6@xjens.apus.local> <457FF0CD.4050606@freesurf.fr>
	<op.tkh10qgw81rjf6@xjens.apus.local>  <4580025D.2040903@freesurf.fr>
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Date: Wed, 13 Dec 2006 14:49:00 +0000
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On mið, 2006-12-13 at 14:38 +0100, Arnaud Lesauvage wrote:
> Jens Schipkowski a écrit :
> > On Wed, 13 Dec 2006 13:23:41 +0100, Arnaud Lesauvage <thewild@freesurf.fr>  
> >> Why is this query better than the other one ? Because it runs the  
> >> "(field IN ('some','other') AND field2 = 'Y')" once and then executes  
> >> the join with the resulting set ?
> > True. The Subselect in FROM clause will be executed once and will be  
> > joined using the condition at where clause. So your condition at t2 is not  
> > executed for each row in t1(2mio records) but for each row in t2(1k  
> > records). And the boolean value is already set during update.
> 
> OK Jens, thanks for clarifying this.
> I thought the planner could guess what to do in such cases.

don't worry, it will.
this is not your problem

gnari