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: 2020-07-26T00:31:49Z
Lists: pgsql-hackers, pgsql-docs 
On Sat, Jul 25, 2020 at 4:56 PM Jeff Davis <pgsql@j-davis.com> wrote:
> I wrote a quick patch to use HyperLogLog to estimate the number of
> groups contained in a spill file. It seems to reduce the
> overpartitioning effect, and is a more principled approach than what I
> was doing before.

This pretty much fixes the issue that I observed with overparitioning.
At least in the sense that the number of partitions grows more
predictably -- even when the number of partitions planned is reduced
the change in the number of batches seems smooth-ish. It "looks nice".

> It does seem to hurt the runtime slightly when spilling to disk in some
> cases. I haven't narrowed down whether this is because we end up
> recursing multiple times, or if it's just more efficient to
> overpartition, or if the cost of doing the HLL itself is significant.

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.

Thanks
-- 
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.