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Re: join selectivity

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: strk(at)refractions(dot)net
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 15:13:03
Message-ID: 28399.1103814783@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
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.

			regards, tom lane

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