On 22/11/12 06:28, Craig James wrote:
On Wed, Nov 21, 2012 at 9:25 AM, Joe Conway
<mail@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
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Or perhaps hints should have the pg version attached, so that they are automatically ignored when the pg version changed? Problem may then become people reluctant to upgrade because their hints relate to a previous version! Sigh...
Even requiring registration of hints and expiring them after a limited time period would not work - as people would simply automate the process of registration & application...
Cheers,
Gavin