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Re: How to avoid redundant Sort operations? (pgsql 7.1.2)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Aris Tsois <atsois(at)dblab(dot)ece(dot)ntua(dot)gr>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: How to avoid redundant Sort operations? (pgsql 7.1.2)
Date: 2002-01-22 16:31:12
Message-ID: 15444.1011717072@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Aris Tsois <atsois(at)dblab(dot)ece(dot)ntua(dot)gr> writes:
> Is there any way to avoid those redundant sort operations?

Information about sort order doesn't propagate up through "subquery
scan" nodes.  There are a couple of reasons for this, one being that
pathkeys aren't returned as part of a completed Plan; there'd have to be
some uglification of the APIs of subquery_planner, grouping_planner, etc
to return the appropriate pathkeys along with the finished plan.
A more subtle problem is that pathkeys of a sub-plan aren't necessarily
meaningful for an outer plan (eg, the variables being sorted on might
not even be visible to the outer query).  I think you'd have to do some
kind of translation to see if you could restate the inner pathkeys in
terms of the subplan's output variables.

			regards, tom lane

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