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

Aris Tsois <atsois@dblab.ece.ntua.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