| From: | Andrew Dunstan <andrew(at)dunslane(dot)net> |
|---|---|
| To: | Claudio Freire <klaussfreire(at)gmail(dot)com> |
| Cc: | Andres Freund <andres(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Craig Ringer <craig(at)2ndquadrant(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, David Greco <David_Greco(at)harte-hanks(dot)com>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Poor performance using CTE |
| Date: | 2012-11-21 16:35:18 |
| Message-ID: | 50AD02C6.8070808@dunslane.net |
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| Lists: | pgsql-performance |
On 11/21/2012 11:32 AM, Claudio Freire wrote:
> On Wed, Nov 21, 2012 at 1:24 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
>> On 2012-11-21 13:16:25 -0300, Claudio Freire wrote:
>>> On Wed, Nov 21, 2012 at 12:32 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>>>> +1
>>>>
>>>> WITH foo AS (SELECT ...) (barrier=on|off)?
>>>>
>>>> 9.3 introduces the syntax, defaulting to on
>>>> 9.4 switches the default to off.
>>> Why syntax? What about a guc?
>>>
>>> collapse_cte_limit?
>> Because there are very good reasons to want to current behaviour. A guc
>> is a global either/or so I don't see it helping much.
> set collapse_cte_limit=8;
> with blah as (blah) select blah;
>
> Not global at all.
>
Then you have to unset it again, which is ugly. You might even want it
applying to *part* of a query, not the whole thing, so this strikes me
as a dead end.
cheers
andrew
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