| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Martijn van Oosterhout <kleptog(at)svana(dot)org> |
| Cc: | "Florian G(dot) Pflug" <fgp(at)phlo(dot)org>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: An Idea for planner hints |
| Date: | 2006-08-09 02:22:26 |
| Message-ID: | 14860.1155090146@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
> My main problem is that selectivity is the wrong measurement. What
> users really want to be able to communicate is:
> 1. If you join tables a and b on x, the number of resulting rows will be
> the number of roows selected from b (since b.x id a foreign key
> referencing a.x).
FWIW, I believe the planner already gets that case right, because a.x
will be unique and it should know that. (Maybe not if the FK is across
a multi-column key, but in principle it should get it right.)
I agree though that meta-knowledge like this is important, and that
standard SQL frequently doesn't provide any adequate way to declare it.
regards, tom lane
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