Date: Mon, 10 Oct 2016 19:07:51 -0700
From: Andres Freund <andres@anarazel.de>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Macro customizable hashtable / bitmapscan &
 aggregation perf
Message-ID: <20161011020751.o2ozug266i5vx7pt@alap3.anarazel.de>
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On 2016-10-10 17:46:22 -0700, Andres Freund wrote:
> > TPC-DS (tpcds.ods)
> > ------------------
> > 
> > In this case, I'd say the results are less convincing. There are quite a few
> > queries that got slower by ~10%, which is well above - for example queries
> > 22 and 67. There are of course queries that got ~10% faster, and in total
> > the patched version executed more queries (so overall the result is slightly
> > positive, but not significantly).
> 
> That's interesting. I wonder whether that's plan changes just due to the
> changing memory estimates, or what's causing that. I'll look into it.

Hm. Based on an initial look those queries aren't planned with any of
the affected codepaths.  Could this primarily be a question of
randomness? Would it perhaps make sense to run the tests in a comparable
order? Looking at tpcds.py and the output files, it seems that the query
order differes between the runs, that can easily explain bigger
difference than the above. For me a scale=1 run creates a database of
approximately 4.5GB, thus with shared_buffers=1GB execution order is
likely to have a significant performance impact.

Greetings,

Andres Freund


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