From: Craig Ringer <craig@2ndQuadrant.com>
To: Gavin Flower <GavinFlower@archidevsys.co.nz>
Cc: Heikki Linnakangas <hlinnakangas@vmware.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Andrew Dunstan <andrew@dunslane.net>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Claudio Freire <klaussfreire@gmail.com>
Cc: Merlin Moncure <mmoncure@gmail.com>
Cc: David Greco <David_Greco@harte-hanks.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Poor performance using CTE
Date: Thu, 22 Nov 2012 08:08:54 +0800
Message-ID: <50AD6D16.9040608@2ndQuadrant.com> (raw)
In-Reply-To: <50AD2BD1.8070206@archidevsys.co.nz>
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<50AD2BD1.8070206@archidevsys.co.nz>
On 11/22/2012 03:30 AM, Gavin Flower wrote:
> On 22/11/12 04:56, Heikki Linnakangas wrote:
>> 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
> WITH FENCE foo AS (SELECT ...)
> default?
That doesn't bind tightly enough to a specific CTE term. Consider:
WITH
FENCE foo AS (SELECT ...),
bar AS (SELECT ...)
SELECT * FROM bar;
Are we fencing just foo? Or all expressions?
--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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To: pgsql-performance@postgresql.org
Cc: craig@2ndQuadrant.com, GavinFlower@archidevsys.co.nz, hlinnakangas@vmware.com, andres@anarazel.de, andrew@dunslane.net, tgl@sss.pgh.pa.us, klaussfreire@gmail.com, mmoncure@gmail.com, David_Greco@harte-hanks.com
Subject: Re: Poor performance using CTE
In-Reply-To: <50AD6D16.9040608@2ndQuadrant.com>
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