Horton

Re: Default setting for enable_hashagg_disk

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Jeff Davis <pgsql@j-davis.com>, Melanie Plageman <melanieplageman@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, pgsql-hackers@postgresql.org
Date: 2020-06-22T15:06:30Z
Lists: pgsql-hackers, pgsql-docs 
On Mon, Jun 22, 2020 at 10:52:37AM -0400, Robert Haas wrote:
> On Wed, Jun 10, 2020 at 2:39 PM Jeff Davis <pgsql@j-davis.com> wrote:
> > The behavior in v13 master is, by default, analagous to Sort or
> > anything else that adapts at runtime to spill. If we had spillable
> > HashAgg the whole time, we wouldn't be worried about #2 at all. But,
> > out of conservatism, I am trying to accommodate users who want an
> > escape hatch, at least for a release or two until users feel more
> > comfortable with disk-based HashAgg.
> >
> > Setting enable_hash_disk=false implements 2(a). This name apparently
> > causes confusion, but it's hard to come up with a better one because
> > the v12 behavior has nuance that's hard to express succinctly. I don't
> > think the names you suggested quite fit, but the idea to use a more
> > interesting GUC value might help express the behavior. Perhaps making
> > enable_hashagg a ternary "enable_hashagg=on|off|avoid_disk"? The word
> > "reject" is too definite for the planner, which is working with
> > imperfect information.
> >
> > In master, there is no explicit way to get 2(b), but you can just set
> > work_mem higher in a lot of cases. If enough people want 2(b), I can
> > add it easily. Perhaps hashagg_overflow=on|off, which would control
> > execution time behavior?
> 
> don't think we do, so I'm very skeptical of the idea that we need 2
> just for this feature. That doesn't feel scalable. I think the right
> number is 0 or 1, and if it's 1, very few people should be changing
> the default. If anything else is the case, then IMHO the feature isn't
> ready to ship.

This was addressed in 92c58fd94801dd5c81ee20e26c5bb71ad64552a8
https://wiki.postgresql.org/index.php?title=PostgreSQL_13_Open_Items&diff=34994&oldid=34993

-- 
Justin

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.