Re: Unfortunate pushing down of expressions below sort

Andres Freund <andres@anarazel.de> writes:
> We push down expressions below a sort node, even though they could be
> evaluated above.

Yeah.  That's a hangover from an ancient decision that sort/limit
would always be applied at the top of the plan tree.  I'm too
lazy to check the details right now, but I think we already relaxed
that in some cases (or maybe it was about evaluating stuff
before/after GROUP BY?).

> That can very substantially increase the space needed for the
> sort.

Could decrease it too, eg if what you are outputting is
md5(some-wide-column).  Not sure we are smart enough to tell
which way is better.

            regards, tom lane