Re: hashagg slowdown due to spill changes

From: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
To: Jeff Davis <pgsql(at)j-davis(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Tomas Vondra <tv(at)fuzzy(dot)cz>, pgsql-hackers(at)lists(dot)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org, Melanie Plageman <melanieplageman(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: hashagg slowdown due to spill changes
Date: 2020-06-13 20:19:41
Message-ID: 20200613201941.wzaqmtju5nrjgj2d@development
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On Sat, Jun 13, 2020 at 11:48:09AM -0700, Jeff Davis wrote:
>On Fri, 2020-06-12 at 17:12 -0700, Andres Freund wrote:
>> Do you think we should tackle this for 13? To me 4cad2534da seems
>> like a
>> somewhat independent improvement to spillable hashaggs.
>We've gone back and forth on this issue a few times, so let's try to
>get some agreement before we revert 4cad2534da. I added Robert because
>he also seemed to think it was a reasonable idea.

I can't speak for Robert, but I haven't expected the extra projection
would be this high. And I agree with Andres it's not very nice we have
to do this even for aggregates with just a handful of groups that don't
need to spill.

In any case, I think we need to address this somehow for v13 - either we
keep the 4cad2534da patch in, or we tweak the cost model to reflect the
extra I/O costs, or we project only when spilling.

I'm not in a position to whip up a patch soon, though :-(


Tomas Vondra
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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