Re: Hints (was Poor performance using CTE)

On 11/21/2012 09:28 AM, Craig James wrote:
>
>
> On Wed, Nov 21, 2012 at 9:25 AM, Joe Conway <mail(at)joeconway(dot)com
> <mailto:mail(at)joeconway(dot)com>> wrote:
>
> 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
> <http://a.id> = b.id <http://b.id>",
> > the user could tell the planner that there are only 10 rows that match
> > the "a.id <http://a.id> = b.id <http://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.
>
>
> Or a three-way choice: Allow, ignore, or generate an error. That would
> allow developers to identify where hints are being used.

+1

Joe

--
Joe Conway
credativ LLC: http://www.credativ.us
Linux, PostgreSQL, and general Open Source
Training, Service, Consulting, & 24x7 Support