Jeff Janes <jeff.janes@gmail.com> writes:
> On Thu, Jul 18, 2013 at 8:04 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 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.
> Couldn't a hash aggregate be superior to a sort one (for the distinct,
> not the order by)?
If it worked at all, it might be superior, but since it doesn't, it ain't.
This isn't really a matter of lack of round tuits, but a deliberate
judgment that it's probably not worth the trouble. Per the comment in
choose_hashed_grouping:
/* * Executor doesn't support hashed aggregation with DISTINCT or ORDER BY * aggregates. (Doing so would
implystoring *all* the input values in * the hash table, and/or running many sorts in parallel, either of which *
seemslike a certain loser.) */
regards, tom lane