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From: Andrew Dunstan <andrew@dunslane.net>
To: Claudio Freire <klaussfreire@gmail.com>
Cc: Andres Freund <andres@2ndquadrant.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Heikki Linnakangas <hlinnakangas@vmware.com>
Cc: Craig Ringer <craig@2ndquadrant.com>
Cc: Merlin Moncure <mmoncure@gmail.com>
Cc: David Greco <David_Greco@harte-hanks.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Poor performance using CTE
Date: Wed, 21 Nov 2012 11:35:18 -0500
Message-ID: <50AD02C6.8070808@dunslane.net> (raw)
In-Reply-To: <CAGTBQpZc78Cmg8nuJ0HvaTRX0Bj5bTNoJUDWMpn8yd8Z9PTtAg@mail.gmail.com>
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On 11/21/2012 11:32 AM, Claudio Freire wrote:
> On Wed, Nov 21, 2012 at 1:24 PM, Andres Freund <andres@2ndquadrant.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@anarazel.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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