Re: SOLVED - RE: Poor performance using CTE

On 11/14/2012 08:17 PM, Craig Ringer wrote:
> 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.


It cuts both ways. I have used CTEs a LOT precisely because this
behaviour lets me get better plans. Without that I'll be back to using
the "offset 0" hack.

cheers

andrew