Date: Fri, 26 Jun 2020 01:18:31 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
Cc: Jeff Davis <pgsql@j-davis.com>, David Rowley <dgrowleyml@gmail.com>,
	Robert Haas <robertmhaas@gmail.com>,
	Justin Pryzby <pryzby@telsasoft.com>,
	Melanie Plageman <melanieplageman@gmail.com>,
	"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Default setting for enable_hashagg_disk
Message-ID: <20200625231831.weorpgrjrlbtcv6d@development>
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On Thu, Jun 25, 2020 at 11:16:23AM -0700, Andres Freund wrote:
>Hi,
>
>On 2020-06-25 10:44:42 -0700, Jeff Davis wrote:
>> There are only two possible paths: HashAgg and Sort+Group, and we need
>> to pick one. If the planner expects one to spill, it is likely to
>> expect the other to spill. If one spills in the executor, then the
>> other is likely to spill, too. (I'm ignoring the case with a lot of
>> tuples and few groups because that doesn't seem relevant.)
>
>There's also ordered index scan + Group. Which will often be vastly
>better than Sort+Group, but still slower than HashAgg.
>
>
>> Imagine that there was only one path available to choose. Would you
>> suggest the same thing, that unexpected spills can exceed work_mem but
>> expected spills can't?
>
>I'm not saying what I propose is perfect, but I've yet to hear a better
>proposal. Given that there *are* different ways to implement
>aggregation, and that we use expected costs to choose, I think the
>assumed costs are relevant.
>

I share Jeff's opinion that this is quite counter-intuitive and we'll
have a hard time explaining it to users.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services