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Re: Planner not using column limit specified for one column for another column equal to first

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Віталій Тимчишин <tivv00(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Planner not using column limit specified for one column for another column equal to first
Date: 2010-04-16 14:19:47
Message-ID: 8534.1271427587@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
=?KOI8-U?B?96bUwcymyiD0yc3eydvJzg==?= <tivv00(at)gmail(dot)com> writes:
> I've thought and someone in this list've told me that this should be done
> automatically.

No, that's not true.  We do make deductions about transitive equalities,
ie, given WHERE a=b AND b=c the planner will infer a=c and use that if
it's helpful.  We don't make deductions about inequalities such as a>c.
In theory there's enough information available to do so, but overall
trying to do that would probably waste more cycles than it would save.
You'd need a lot of expensive new planner infrastructure, and in the
vast majority of queries it wouldn't produce anything very helpful.

As was pointed out, even if we had such logic it wouldn't apply in this
example, because the equality conditions aren't real equalities but
OUTER JOIN conditions.

			regards, tom lane

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