| From: | strk(at)refractions(dot)net |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Mark Cave-Ayland <m(dot)cave-ayland(at)webbased(dot)co(dot)uk>, pgsql-hackers(at)postgresql(dot)org, postgis-devel(at)postgis(dot)refractions(dot)net |
| Subject: | Re: join selectivity |
| Date: | 2004-12-23 15:41:47 |
| Message-ID: | 20041223154147.GC4535@freek.keybit.net |
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| Lists: | pgsql-hackers |
On Thu, Dec 23, 2004 at 10:13:03AM -0500, Tom Lane wrote:
> strk(at)refractions(dot)net writes:
> > On Thu, Dec 23, 2004 at 10:01:33AM -0500, Tom Lane wrote:
> >> Right. This amounts to assuming that the join conditions and the
> >> restriction conditions are independent, which of course is bogus,
> >> but we really don't have enough information to do better.
>
> > Doesn't JOINSEL have access to RESTRICTSEL output for REL1 and REL2 ?
>
> You could probably compare the fields of the RelOptInfo structures,
> but what are you going to do with it? AFAICS you *should not* make
> the join selectivity depend on that.
So it should NOT depend on full number of rows either, is this right ?
--strk;
>
> regards, tom lane
>
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