On Tue, Feb 11, 2003 at 10:41:53 -0500, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Greg Stark <gsstark@mit.edu> writes:
> > So one of the items on the TODO list is "Add hash for evaluating GROUP BY
> > aggregates (Tom)"
>
> It's done in CVS tip ... give it a try.
>
> > The neat thing is that hash aggregates would allow grouping on data types that
> > have = operators but no useful < operator.
>
> Hm. Right now I think that would barf on you, because the parser wants
> to find the '<' operator to label the grouping column with, even if the
> planner later decides not to use it. It'd take some redesign of the
> query data structure (specifically SortClause/GroupClause) to avoid that.
I think another issue is that for some = operators you still might not
be able to use a hash. I would expect the discussion for hash joins in
http://developer.postgresql.org/docs/postgres/xoper-optimization.html
would to hash aggregates as well.