Re: Hints (was Poor performance using CTE)

From: Joe Conway <mail(at)joeconway(dot)com>
To: Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>
Cc: Kevin Grittner <kgrittn(at)mail(dot)com>, Craig Ringer <craig(at)2ndQuadrant(dot)com>, Craig James <cjames(at)emolecules(dot)com>, 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
Subject: Re: Hints (was Poor performance using CTE)
Date: 2012-11-21 17:25:52
Message-ID: 50AD0EA0.7070009@joeconway.com
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On 11/21/2012 08:05 AM, Heikki Linnakangas wrote:
> Rather than telling the planner what to do or not to do, I'd much rather
> have hints that give the planner more information about the tables and
> quals involved in the query. A typical source of bad plans is when the
> planner gets its cost estimates wrong. So rather than telling the
> planner to use a nested loop join for "a INNER JOIN b ON a.id = b.id",
> the user could tell the planner that there are only 10 rows that match
> the "a.id = b.id" qual. That gives the planner the information it needs
> to choose the right plan on its own. That kind of hints would be much
> less implementation specific and much more likely to still be useful, or
> at least not outright counter-productive, in a future version with a
> smarter planner.
>
> You could also attach that kind of hints to tables and columns, which
> would be more portable and nicer than decorating all queries.

I like this idea, but also think that if we have a syntax to allow
hints, it would be nice to have a simple way to ignore all hints (yes, I
suppose I'm suggesting yet another GUC). That way after sprinkling your
SQL with hints, you could easily periodically (e.g. after a Postgres
upgrade) test what would happen if the hints were removed.

Joe
--
Joe Conway
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