From: Andres Freund <andres@anarazel.de>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: Jeff Davis <pgsql@j-davis.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: David Rowley <dgrowleyml@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 15:48:24 -0700
Message-ID: <85F9F4F0-918A-457F-BC9E-F2AC2EFC1F86@anarazel.de> (raw)
In-Reply-To: <20200625224422.GA9653@alvherre.pgsql>
References: <20200625224422.GA9653@alvherre.pgsql>
Hi,
On June 25, 2020 3:44:22 PM PDT, Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
>On 2020-Jun-25, Andres Freund wrote:
>
>> > What are people doing for those cases already? Do we have an
>> > real-world queries that are a problem in PG 13 for this?
>>
>> I don't know about real world, but it's pretty easy to come up with
>> examples.
>>
>> query:
>> SELECT a, array_agg(b) FROM (SELECT generate_series(1, 10000)) a(a),
>(SELECT generate_series(1, 10000)) b(b) GROUP BY a HAVING
>array_length(array_agg(b), 1) = 0;
>>
>> work_mem = 4MB
>>
>> 12 18470.012 ms
>> HEAD 44635.210 ms
>>
>> HEAD causes ~2.8GB of file IO, 12 doesn't cause any. If you're IO
>> bandwidth constrained, this could be quite bad.
>
>... however, you can pretty much get the previous performance back by
>increasing work_mem. I just tried your example here, and I get 32
>seconds of runtime for work_mem 4MB, and 13.5 seconds for work_mem 1GB
>(this one spills about 800 MB); if I increase that again to 1.7GB I get
>no spilling and 9 seconds of runtime. (For comparison, 12 takes 15.7
>seconds regardless of work_mem).
>
>My point here is that maybe we don't need to offer a GUC to explicitly
>turn spilling off; it seems sufficient to let users change work_mem so
>that spilling will naturally not occur. Why do we need more?
That's not really a useful escape hatch, because I'll often lead to other nodes using more memory.
Andres
--
Sent from my Android device with K-9 Mail. Please excuse my brevity.
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Cc: andres@anarazel.de, alvherre@2ndquadrant.com, bruce@momjian.us, pgsql@j-davis.com, robertmhaas@gmail.com, dgrowleyml@gmail.com, pryzby@telsasoft.com, melanieplageman@gmail.com, tomas.vondra@2ndquadrant.com, pgsql-hackers@postgresql.org
Subject: Re: Default setting for enable_hashagg_disk
In-Reply-To: <85F9F4F0-918A-457F-BC9E-F2AC2EFC1F86@anarazel.de>
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