Re: Poor performance using CTE

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