From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Peter Geoghegan <pg@bowt.ie>, Stephen Frost <sfrost@snowman.net>,
        Jeff Davis <pgsql@j-davis.com>, Andres Freund <andres@anarazel.de>,
        Bruce Momjian <bruce@momjian.us>, Robert Haas <robertmhaas@gmail.com>,
        David Rowley <dgrowleyml@gmail.com>,
        Justin Pryzby <pryzby@telsasoft.com>,
        Melanie Plageman <melanieplageman@gmail.com>,
        Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Default setting for enable_hashagg_disk
In-reply-to: <20200710211026.GA4375@alvherre.pgsql>
References: <20200710211026.GA4375@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
	message dated "Fri, 10 Jul 2020 17:10:26 -0400"
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Date: Fri, 10 Jul 2020 17:28:15 -0400
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> On 2020-Jul-10, Peter Geoghegan wrote:
>> * The maximum allowable value is 100.0, to protect users from
>> accidentally setting hash_mem_multiplier to a value intended to work
>> like a work_mem-style KB value (you can't provide an absolute value
>> like that directly). This maximum is absurdly high.

> I'm not sure about this bit; sounds a bit like what has been qualified
> as "nannyism" elsewhere.  Suppose I want to give a hash table 2GB of
> memory for whatever reason.  If my work_mem is default (4MB) then I
> cannot possibly achieve that without altering both settings.
> So I propose that maybe we do want a maximum value, but if so it should
> be higher than what you propose.  I think 10000 is acceptable in that it
> doesn't get in the way.

I was kind of thinking 1000 as the limit ;-).  In any case, the code
will need to internally clamp the product to not exceed whatever the
work_mem physical limit is these days.

			regards, tom lane