Horton

Re: Default setting for enable_hashagg_disk (hash_mem)

Alvaro Herrera <alvherre@2ndquadrant.com>

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Bruce Momjian <bruce@momjian.us>
Cc: Peter Geoghegan <pg@bowt.ie>, Justin Pryzby <pryzby@telsasoft.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, David Rowley <dgrowleyml@gmail.com>, Jeff Davis <pgsql@j-davis.com>, Melanie Plageman <melanieplageman@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-07-03T19:50:02Z
Lists: pgsql-hackers, pgsql-docs 
On 2020-Jul-03, Bruce Momjian wrote:

> Well, the bottom line is that we are designing features during beta.

Well, we're designing a way for users to interact the new feature.
The feature itself is already in, and it works well in general terms.  I
expect that the new behavior is a win in the majority of cases, and the
problem being discussed here will only manifest as a regression in
corner cases.  (I don't have data to back this up, but if this weren't
the case we would have realized much earlier).

It seem to me we're designing a solution to a problem that was found
during testing, which seems perfectly acceptable to me.  I don't see
grounds for reverting the behavior and I haven't seen anyone suggesting
that it would be an appropriate solution to the issue.

> If we add a new behavior to PG 13, we then have the pre-PG 13 behavior,
> the pre-patch behavior, and the post-patch behavior.  How are people
> supposed to test all of that?

They don't have to.  We tell them that we added some new tweak for a new
pg13 feature in beta3 and that's it.

> Add to that that some don't even feel we
> need a new behavior, which is delaying any patch from being applied.

If we don't need any new behavior, then we would just close the open
item and call the current state good, no?

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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.