Re: AGG_PLAIN thinks sorts are free

On Fri, Jul 19, 2013 at 8:34 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
> > AGG_PLAIN sometimes does sorts, but it thinks they are free. Also, under
> > explain analyze it does not explicitly report whether the sort was
> external
> > or not, nor report the disk or memory usage, the way other sorts do. I
> > don't know if those two things are related or not.
>
> DISTINCT (and also ORDER BY) properties of aggregates are implemented
> at runtime; the planner doesn't really do anything about them, except
> suppress the choice it might otherwise make of using hashed aggregation.
> Since the behavior is entirely local to the Agg plan node, it's also
> not visible to the EXPLAIN ANALYZE machinery.
>
> Arguably we should have the planner add on some cost factor for such
> aggregates, but that would have no effect whatever on the current level
> of plan, and could only be useful if this was a subquery whose cost
> would affect choices in an outer query level. Which is a case that's
> pretty few and far between AFAIK (do you have a real-world example where
> it matters?). So it's something that hasn't gotten to the top of
> anybody's to-do list.
>
> An arguably more useful thing to do would be to integrate this behavior
> into the planner more completely, so that (for instance) if only one
> aggregate had ORDER BY then we would make the underlying query produce
> that order instead of implementing a sort locally in the Agg node.
>

Slight modification would be *all* aggregates having same ORDER BY clause.
I think it would be easy given that we already influence the sortedness of
underlying query result in planner.

> That hasn't risen to the top of the to-do list either, as yet.
>
> regards, tom lane
>
>
> --
> Sent via pgsql-hackers mailing list (pgsql-hackers(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-hackers
>

--
Best Wishes,
Ashutosh Bapat
EntepriseDB Corporation
The Postgres Database Company