Re: Poor performance using CTE

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: Claudio Freire <klaussfreire(at)gmail(dot)com>
Cc: Andrew Dunstan <andrew(at)dunslane(dot)net>, 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 16:34:50
Message-ID: 20121121163450.GB6268@awork2.anarazel.de
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On 2012-11-21 13:32:45 -0300, 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.

Not very manageable though. And it doesn't help if you need both in a
query which isn't actually that unlikely.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

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