Horton

Re: Default setting for enable_hashagg_disk

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Bruce Momjian <bruce@momjian.us>,Jeff Davis <pgsql@j-davis.com>,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>
Date: 2020-06-25T22:48:24Z
Lists: pgsql-hackers, pgsql-docs 
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.

Commits

  1. Add hash_mem_multiplier GUC.

  2. HashAgg: use better cardinality estimate for recursive spilling.

  3. Remove hashagg_avoid_disk_plan GUC.

  4. Doc fixup for hashagg_avoid_disk_plan GUC.

  5. Rework HashAgg GUCs.

  6. Disk-based Hash Aggregation.

  7. Implement partition-wise grouping/aggregation.

  8. Defer creation of partially-grouped relation until it's needed.