Message-ID: <50A442A8.10803@2ndQuadrant.com>
Date: Thu, 15 Nov 2012 09:17:28 +0800
From: Craig Ringer <craig@2ndQuadrant.com>
Organization: 2nd Quadrant
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: David Greco <David_Greco@harte-hanks.com>,
	Andrew Dunstan <andrew@dunslane.net>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: SOLVED - RE: Poor performance using CTE
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On 11/15/2012 12:29 AM, Tom Lane wrote:
> David Greco <David_Greco@harte-hanks.com> writes:
>> Thanks, that did the trick. Though I'm still not clear as to why. 
> PG treats WITH as an optimization fence --- the WITH query will be
> executed pretty much as-is.  It may be that Oracle flattens the query
> somehow; though if you're using black-box functions in both cases,
> it's not obvious where the optimizer could get any purchase that way.
>

I was looking through the latest spec drafts I have access to and
couldn't find any reference to Pg's optimisation-fence-for-CTEs
behaviour being required by the standard, though I've repeatedly seen it
said that there is such a requirement.

Do you know where it's specified?

All I can see is that the optimised result must have the same effect as
the original. That'd mean that wCTEs and CTE terms that use VOLATILE
functions or functions with side-effects couldn't be optimised into
other queries. Simple CTEs could be, though, and there are times I've
really wished I could use a CTE but I've had to use a set-returning
subquery to get reasonable plans.

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