Horton

Re: Default setting for enable_hashagg_disk

Greg Stark <stark@mit.edu>

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From: Greg Stark <stark@mit.edu>
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Jeff Davis <pgsql@j-davis.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2020-06-12T03:35:19Z
Lists: pgsql-hackers, pgsql-docs 
On Thu, 9 Apr 2020 at 13:24, Justin Pryzby <pryzby@telsasoft.com> wrote:

> On Thu, Apr 09, 2020 at 01:48:55PM +0200, Tomas Vondra wrote:
>
> > It it really any different from our enable_* GUCs? Even if you do e.g.
> > enable_sort=off, we may still do a sort. Same for enable_groupagg etc.
>
> Those show that the GUC was disabled by showing disable_cost.  That's
> what's
> different about this one.
>

Fwiw in the past this was seen not so much as a positive thing but a bug to
be fixed. We've talked about carrying a boolean "disabled plan" flag which
would be treated as a large cost penalty but not actually be added to the
cost in the plan.

The problems with the disable_cost in the cost are (at least):

1) It causes the resulting costs to be useless for comparing the plan costs
with other plans.

2) It can cause other planning decisions to be distorted in strange
non-linear ways.


-- 
greg

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.