Re: SOLVED - RE: Poor performance using CTE

On 11/15/2012 12:29 AM, Tom Lane wrote:
> David Greco <David_Greco(at)harte-hanks(dot)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/
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