On Wed, Nov 21, 2012 at 9:25 AM, Joe Conway <m...@joeconway.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 = 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. > Or a three-way choice: Allow, ignore, or generate an error. That would allow developers to identify where hints are being used. Craig > > Joe > -- > Joe Conway > credativ LLC: http://www.credativ.us > Linux, PostgreSQL, and general Open Source > Training, Service, Consulting, & 24x7 Support > > >
