Horton

Re: Default setting for enable_hashagg_disk

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Tom Lane <tgl@sss.pgh.pa.us>, Alvaro Herrera <alvherre@2ndquadrant.com>, David Rowley <dgrowleyml@gmail.com>, Stephen Frost <sfrost@snowman.net>, Andres Freund <andres@anarazel.de>, Bruce Momjian <bruce@momjian.us>, Robert Haas <robertmhaas@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Melanie Plageman <melanieplageman@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2020-07-26T00:52:11Z
Lists: pgsql-hackers, pgsql-docs 
On Sat, Jul 25, 2020 at 5:31 PM Peter Geoghegan <pg@bowt.ie> wrote:
> I'm glad that this better principled approach is possible. It's hard
> to judge how much of a problem this really is, though. We'll need to
> think about this aspect some more.

BTW, your HLL patch ameliorates the problem with my extreme "sorted vs
random input" test case from this morning [1] (the thing that I just
discussed with Tomas). Without the HLL patch the sorted case had 2424
batches. With the HLL patch it has 20. That at least seems like a
notable improvement.

[1] https://postgr.es/m/CAH2-Wz=ur7MQKpaUZJP=Adtg0TPMx5M_WoNE=ke2vUU=amdjPQ@mail.gmail.com
-- 
Peter Geoghegan

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.