| From: | Craig Ringer <craig(at)2ndQuadrant(dot)com> |
|---|---|
| To: | Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz> |
| Cc: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Andrew Dunstan <andrew(at)dunslane(dot)net>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Claudio Freire <klaussfreire(at)gmail(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: | 2012-11-22 00:42:51 |
| Message-ID: | 50AD750B.8010108@2ndQuadrant.com |
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| Lists: | pgsql-performance |
On 11/22/2012 08:38 AM, Gavin Flower wrote:
> I suspect most people are blissfully unaware of CTE's being fenced, or
> at least not really sure what it means. So I suspect NOT FENCE would
> be the better default.
It's also probably more standard, and a better fit with what other DBs do.
Pg would still need to detect conditions like the use of functions with
side effects or (obviously) INSERT/UPDATE/DELETE wCTEs and not push
conditions down into them / pull conditions up from them, etc. That's
how I read the standard, though; must have the same effect as if the
queries were executed as written, so Pg is free to transform them so
long as it doesn't change the results.
--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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