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. 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. "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. -- Justin
