Re: queries with DISTINCT / GROUP BY giving different plans

"Tomas Vondra" <tv(at)fuzzy(dot)cz> writes:
> I've run into a strange plan difference on 9.1.9 - the first query does
> "DISTINCT" by doing a GROUP BY on the columns (both INT). ...
> Now, this takes ~45 seconds to execute, but after rewriting the query to
> use the regular DISTINCT it suddenly switches to HashAggregate with ~1/3
> the cost (although it produces the same output, AFAIK), and it executes in
> ~15 seconds.

[ scratches head... ] I guess you're running into some corner case where
choose_hashed_grouping and choose_hashed_distinct make different choices.
It's going to be tough to debug without a test case though. I couldn't
reproduce the behavior in a few tries here.

> BTW I can't test this on 9.2 or 9.3 easily, as this is our production
> environment and I can't just export the data. I've tried to simulate this
> but so far no luck.

I suppose they won't yet you step through those two functions with a
debugger either ...

regards, tom lane