Message-ID: <50ACF991.3020603@vmware.com>
Date: Wed, 21 Nov 2012 17:56:01 +0200
From: Heikki Linnakangas <hlinnakangas@vmware.com>
Organization: VMware
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To: Gavin Flower <GavinFlower@archidevsys.co.nz>
CC: Andres Freund <andres@anarazel.de>, Andrew Dunstan <andrew@dunslane.net>,
	Tom Lane <tgl@sss.pgh.pa.us>, 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
References: 
 <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> <50ACD166.1000507@vmware.com>
	<50ACE970.7050701@dunslane.net> <3396.1353509947@sss.pgh.pa.us>
	<50ACF16C.2020902@dunslane.net>
	<20121121153209.GA1271@awork2.anarazel.de>
	<50ACF658.10902@archidevsys.co.nz>
In-Reply-To: <50ACF658.10902@archidevsys.co.nz>
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On 21.11.2012 17:42, Gavin Flower wrote:
> On 22/11/12 04:32, Andres Freund wrote:
>> On 2012-11-21 10:21:16 -0500, Andrew Dunstan wrote:
>>> I wasn't talking about removing it. My point was that if the
>>> optimization
>>> fence around CTEs is removed a lot of people will need to rework apps
>>> where
>>> they have used them for that purpose. And I continue to think that
>>> spelling
>>> it "OFFSET 0" is horribly obscure.
>> +1

FWIW, I'm happy with "OFFSET 0". Granted, it's pretty obscure, but 
that's what we've historically recommended, and it's pretty ugly to have 
to specify a fence like that in the first place. Whenever you have to 
resort to it, you ought have a comment in the query explaining why you 
need to force the planner like that, anyway.

>> WITH foo AS (SELECT ...) (barrier=on|off)?
>>
>> 9.3 introduces the syntax, defaulting to on
>> 9.4 switches the default to off.
>
> WITH foo AS (SELECT ...) (fence=on|off)?
>
> WITH foo AS (SELECT ...) (optimisation_fence=on|off)?

If we are to invent a new syntax for this, can we please come up with 
something that's more widely applicable than just the WITH syntax. 
Something that you could use to replace OFFSET 0 in a subquery, too.

- Heikki