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From: Andres Freund <andres@anarazel.de>
To: Jeff Davis <pgsql@j-davis.com>
Cc: David Rowley <dgrowleyml@gmail.com>
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: Thu, 25 Jun 2020 09:37:46 -0700
Message-ID: <20200625163746.laoepc7esanptvxt@alap3.anarazel.de> (raw)
In-Reply-To: <b1e3b762196bb6288a63c235f78344ead900ea6d.camel@j-davis.com>
References: <CAAKRu_aisiENMsPM2gC4oUY1hHG3yrCwY-fXUg22C6_MJUwQdA@mail.gmail.com>
	<20200610021544.GA14879@telsasoft.com>
	<8bff2e4e8020c3caa16b61a46918d21b573eaf78.camel@j-davis.com>
	<CA+TgmoagMLfruY4y_40oKLmFFdBRVSGraZsTa5ss9wH546EYhQ@mail.gmail.com>
	<ee1a7be071349b93c01ffb9ef4cf3a6830d9e780.camel@j-davis.com>
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	<129c8f1739957589584d9525e1d8782eae3f412f.camel@j-davis.com>
	<CAApHDvqFZikXhAGW=UKZKq1_FzHy+XzmUzAJiNj6RWyTHH4UfA@mail.gmail.com>
	<20200624191433.5gnqgrxfmucexldm@alap3.anarazel.de>
	<b1e3b762196bb6288a63c235f78344ead900ea6d.camel@j-davis.com>

On 2020-06-25 09:24:52 -0700, Jeff Davis wrote:
> On Wed, 2020-06-24 at 12:14 -0700, Andres Freund wrote:
> > E.g. if the plan isn't expected to spill,
> > only spill at 10 x work_mem or something like that.
> 
> Let's say you have work_mem=32MB and a query that's expected to use
> 16MB of memory. In reality, it uses 64MB of memory. So you are saying
> this query would get to use all 64MB of memory, right?
> 
> But then you run ANALYZE. Now the query is (correctly) expected to use
> 64MB of memory. Are you saying this query, executed again with better
> stats, would only get to use 32MB of memory, and therefore run slower?

Yes. I think that's ok, because it was taken into account from a costing
perspective int he second case.





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