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
