Re: Poor performance using CTE

On 11/22/2012 03:30 AM, Gavin Flower wrote:
> On 22/11/12 04:56, Heikki Linnakangas wrote:
>> 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
> WITH FENCE foo AS (SELECT ...)
> default?
That doesn't bind tightly enough to a specific CTE term. Consider:

WITH
FENCE foo AS (SELECT ...),
bar AS (SELECT ...)
SELECT * FROM bar;

Are we fencing just foo? Or all expressions?

--
Craig Ringer http://www.2ndQuadrant.com/
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