Re: Default setting for enable_hashagg_disk

On Tue, Apr 07, 2020 at 05:39:01PM -0500, Justin Pryzby wrote:
>On Tue, Apr 07, 2020 at 11:20:46AM -0700, Jeff Davis wrote:
>> The enable_hashagg_disk GUC, if set to true, chooses HashAgg based on
>> costing. If false, it only generates a HashAgg path if it thinks it will fit
>> in work_mem, similar to the old behavior (though it wlil now spill to disk if
>> the planner was wrong about it fitting in work_mem).  The current default is
>> true.
>
>Are there any other GUCs that behave like that ?  It's confusing to me when I
>see "Disk Usage: ... kB", despite setting it to "disable", and without the
>usual disable_cost.  I realize that postgres chose the plan on the hypothesis
>that it would *not* exceed work_mem, and that spilling to disk is considered
>preferable to ignoring the setting, and that "going back" to planning phase
>isn't a possibility.
>

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.

>template1=# explain (analyze, costs off, summary off) SELECT a, COUNT(1) FROM generate_series(1,999999) a GROUP BY 1
;
> HashAggregate (actual time=1370.945..2877.250 rows=999999 loops=1)
>   Group Key: a
>   Peak Memory Usage: 5017 kB
>   Disk Usage: 22992 kB
>   HashAgg Batches: 84
>   ->  Function Scan on generate_series a (actual time=314.507..741.517 rows=999999 loops=1)
>
>A previous version of the docs said this, which I thought was confusing, and you removed it.
>But I guess this is the behavior it was trying to .. explain.
>
>+      <term><varname>enable_hashagg_disk</varname> (<type>boolean</type>)
>+        ... This only affects the planner choice;
>+        execution time may still require using disk-based hash
>+        aggregation. The default is <literal>on</literal>.
>
>I suggest that should be reworded and then re-introduced, unless there's some
>further behavior change allowing the previous behavior of
>might-exceed-work-mem.
>

Yeah, it would be good to mention this is a best-effort setting.

>"This setting determines whether the planner will elect to use a hash plan
>which it expects will exceed work_mem and spill to disk.  During execution,
>hash nodes which exceed work_mem will spill to disk even if this setting is
>disabled.  To avoid spilling to disk, either increase work_mem (or set
>enable_hashagg=off)."
>
>For sure the release notes should recommend re-calibrating work_mem.
>

I don't follow. Why would the recalibrating be needed?

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services