From: Tom Lane <tgl@sss.pgh.pa.us>
To: "Tomas Vondra" <tv@fuzzy.cz>
cc: pgsql-performance@postgresql.org
Subject: Re: queries with DISTINCT / GROUP BY giving different plans
In-reply-to: <3a56c4d2c0b2daa11a625c2983823901.squirrel@sq.gransy.com>
References: <3a56c4d2c0b2daa11a625c2983823901.squirrel@sq.gransy.com>
Comments: In-reply-to "Tomas Vondra" <tv@fuzzy.cz>
 message dated "Wed, 14 Aug 2013 17:33:53 +0200"
Date: Wed, 14 Aug 2013 14:35:14 -0400
Message-ID: <11586.1376505314@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

"Tomas Vondra" <tv@fuzzy.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


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