Message-ID: <520E7B71.6060704@fuzzy.cz>
Date: Fri, 16 Aug 2013 21:20:17 +0200
From: Tomas Vondra <tv@fuzzy.cz>
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To: pgsql-performance@postgresql.org
Subject: Re: queries with DISTINCT / GROUP BY giving different plans
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On 14.8.2013 20:35, Tom Lane wrote:
> "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 ...

I've managed to get the data to a different machine, and I've spent some
time on debugging it. It seems that the difference is in evaluating
hashentrysize - while


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