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From: Craig Ringer <craig@2ndQuadrant.com>
To: 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: Wed, 21 Nov 2012 07:38:26 +0800
Message-ID: <50AC1472.3050607@2ndQuadrant.com> (raw)
In-Reply-To: <CAGTBQpbf9bdJ3gjcYdw2FuPmN=dqx+wE7opsCp515cEndpGDYw@mail.gmail.com>
References: <187F6C10D2931A4386EE8E58E13857F612917154@BLUPRD0811MB413.namprd08.prod.outlook.com>
	<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>

On 11/21/2012 12:06 AM, Claudio Freire wrote:
> I meant for postgres to do automatically. Rewriting as a join wouldn't
> work as an optimization fence the way we're used to, but pushing
> constraints upwards can only help (especially if highly selective).
Because people are now used to using CTEs as query hints, it'd probably
cause performance regressions in working queries. Perhaps more
importantly, Pg would have to prove that doing so didn't change queries
that invoked functions with side-effects to avoid changing the results
of currently valid queries.

-- 
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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