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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Peter Geoghegan <pg@bowt.ie>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: David Rowley <dgrowleyml@gmail.com>
Cc: Stephen Frost <sfrost@snowman.net>
Cc: Andres Freund <andres@anarazel.de>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: Default setting for enable_hashagg_disk
Date: Sat, 18 Jul 2020 21:15:33 -0400
Message-ID: <32871.1595121333@sss.pgh.pa.us> (raw)
In-Reply-To: <571473265c5d5309f4012855ebd2131c136073ab.camel@j-davis.com>
References: <5cf74833a830e76cc7cc7506aa77bd6ab0c94f4c.camel@j-davis.com>
	<20200713164736.GA25726@alvherre.pgsql>
	<CAH2-WzmaJG4QYHq5LR+ZXEf1JMVcTzT9pEOeMFg0uuGXYDvgzQ@mail.gmail.com>
	<9e4a4cffe3eee0543e5b94c90821d39e03b81d29.camel@j-davis.com>
	<CAH2-Wz=qBF1QyXKdB6YNV993WpQ0wj-x4h8r9yjYUwQRCMkLjA@mail.gmail.com>
	<6d0aba397625df79bdc3ca85adc11b9e48ecb0da.camel@j-davis.com>
	<4118680.1595097043@sss.pgh.pa.us>
	<571473265c5d5309f4012855ebd2131c136073ab.camel@j-davis.com>

Jeff Davis <pgsql@j-davis.com> writes:
> What is your opinion about pessimizing the HashAgg disk costs (not
> affecting HashAgg plans expected to stay in memory)? Tomas Vondra
> presented some evidence that Sort had some better IO patterns in some
> cases that weren't easily reflected in a principled way in the cost
> model.

Hm, was that in some other thread?  I didn't find any such info
in a quick look through this one.

> That would lessen the number of changed plans, but we could easily
> remove the pessimization without controversy later if it turned out to
> be unnecessary, or if we further optimize HashAgg IO.

Trying to improve our cost models under-the-hood seems like a
perfectly reasonable activity to me.

			regards, tom lane





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