From: Bruce Momjian <bruce@momjian.us>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: David Rowley <dgrowleyml@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: Default setting for enable_hashagg_disk
Date: Thu, 25 Jun 2020 15:24:42 -0400
Message-ID: <20200625192442.GD12486@momjian.us> (raw)
In-Reply-To: <fae0c8abc541f90c9078c9ffaa2904caad6a20b7.camel@j-davis.com>
References: <8bff2e4e8020c3caa16b61a46918d21b573eaf78.camel@j-davis.com>
<CA+TgmoagMLfruY4y_40oKLmFFdBRVSGraZsTa5ss9wH546EYhQ@mail.gmail.com>
<ee1a7be071349b93c01ffb9ef4cf3a6830d9e780.camel@j-davis.com>
<CA+TgmoY4-zSMFf8XbpO6uRsMX4vNPPLXvtajAZpG3f9eoEyjdA@mail.gmail.com>
<129c8f1739957589584d9525e1d8782eae3f412f.camel@j-davis.com>
<CAApHDvqFZikXhAGW=UKZKq1_FzHy+XzmUzAJiNj6RWyTHH4UfA@mail.gmail.com>
<20200624090628.GA19640@momjian.us>
<20200624123843.GX4107@telsasoft.com>
<2214502.1593019796@sss.pgh.pa.us>
<fae0c8abc541f90c9078c9ffaa2904caad6a20b7.camel@j-davis.com>
On Thu, Jun 25, 2020 at 11:10:58AM -0700, Jeff Davis wrote:
> On Wed, 2020-06-24 at 13:29 -0400, Tom Lane wrote:
> > If we feel we need something to let people have the v12 behavior
> > back, let's have
> > (1) enable_hashagg on/off --- controls planner, same as it ever was
> > (2) enable_hashagg_spill on/off --- controls executor by disabling
> > spill
> >
> > But I'm not really convinced that we need (2).
>
> If we're not going to have a planner GUC, one alternative is to just
> penalize the disk costs of HashAgg for a release or two. It would only
> affect the cost of HashAgg paths that are expected to spill, which
> weren't even generated in previous releases.
>
> In other words, multiply the disk costs by enough that the planner will
> usually not choose HashAgg if expected to spill unless the average
> group size is quite large (i.e. there are a lot more tuples than
> groups, but still enough groups to spill).
Well, the big question is whether this costing is actually more accurate
than what we have now. What I am hearing is that spilling hash agg is
expensive, so whatever we can do to reflect the actual costs seems like
a win. If it can be done, it certainly seems better than a cost setting
few people will use.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EnterpriseDB https://enterprisedb.com
The usefulness of a cup is in its emptiness, Bruce Lee
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Cc: bruce@momjian.us, pgsql@j-davis.com, tgl@sss.pgh.pa.us, pryzby@telsasoft.com, dgrowleyml@gmail.com, robertmhaas@gmail.com, melanieplageman@gmail.com, tomas.vondra@2ndquadrant.com, pgsql-hackers@postgresql.org
Subject: Re: Default setting for enable_hashagg_disk
In-Reply-To: <20200625192442.GD12486@momjian.us>
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