Date: Mon, 29 Jun 2020 13:31:40 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	Robert Haas <robertmhaas@gmail.com>,
	Andres Freund <andres@anarazel.de>,
	David Rowley <dgrowleyml@gmail.com>, Jeff Davis <pgsql@j-davis.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: <20200629173140.GE17529@momjian.us>
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On Mon, Jun 29, 2020 at 10:20:14AM -0700, Peter Geoghegan wrote:
> I have no reason to believe that the planner is any more or any less
> likely to conclude that the hash table will fit in memory in v13 as
> things stand (I don't know if the BufFile issue matters).
> 
> In general, grouping estimates probably aren't very good compared to
> join estimates. I imagine that in either v12 or v13 the planner is
> likely to incorrectly believe that it'll all fit in memory fairly
> often. v12 was much too permissive about what could happen. But v13 is
> too conservative.

FYI, we have improved planner statistics estimates for years, which must
have affected node spill behavior on many node types (except hash_agg),
and don't remember any complaints about it.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee