Message-ID: <50ACD166.1000507@vmware.com>
Date: Wed, 21 Nov 2012 15:04:38 +0200
From: Heikki Linnakangas <hlinnakangas@vmware.com>
Organization: VMware
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: Craig Ringer <craig@2ndQuadrant.com>,
	Claudio Freire <klaussfreire@gmail.com>,
	Merlin Moncure <mmoncure@gmail.com>,
	David Greco <David_Greco@harte-hanks.com>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Poor performance using CTE
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	<CAHyXU0xeEVX2P=HaNxPiKayEhkWp3GobgAXCQPrfsqbawXVV-w@mail.gmail.com>
	<CAGTBQpZ_Za-XSXvmPUx3F7wgeX=HB1xUpDsn+1DXY3a87YJLBQ@mail.gmail.com>
	<CAHyXU0z4oUss50vhH_wGZ-HERQe9wCMSf1nMTGNddXeCoKvVVA@mail.gmail.com>
	<CAGTBQpbf9bdJ3gjcYdw2FuPmN=dqx+wE7opsCp515cEndpGDYw@mail.gmail.com>
	<50AC1472.3050607@2ndQuadrant.com> <18123.1353455585@sss.pgh.pa.us>
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On 21.11.2012 01:53, Tom Lane wrote:
> I think the more interesting question is what cases wouldn't be covered
> by such a rule.  Typically you need to use OFFSET 0 in situations where
> the planner has guessed wrong about costs or rowcounts, and I think
> people are likely using WITH for that as well.  Should we be telling
> people that they ought to insert OFFSET 0 in WITH queries if they want
> to be sure there's an optimization fence?

Yes, I strongly feel that we should. Writing a query using WITH often 
makes it more readable. It would be a shame if people have to refrain 
from using it, because the planner treats it as an optimization fence.

- Heikki