Israel Brewster <ijbrews...@alaska.edu> writes:
> In looking at the explain analyze output, I noticed that it had an “external
> merge Disk” sort going on, accounting for about 1 second of the runtime
> (explain analyze output here: https://explain.depesz.com/s/jx0q
> <https://explain.depesz.com/s/jx0q>). Since the machine has plenty of RAM
> available, I went ahead and increased the work_mem parameter. Whereupon the
> query plan got much simpler, and performance of said query completely tanked,
> increasing to about 15.5 seconds runtime (https://explain.depesz.com/s/Kl0S
> <https://explain.depesz.com/s/Kl0S>), most of which was in a HashAggregate.
> How can I fix this? Thanks.
Well, the brute-force way not to get that plan is "set enable_hashagg =
false". But it'd likely be a better idea to try to improve the planner's
rowcount estimates. The problem here seems to be lack of stats for
either "time_bucket('1 week', read_time)" or "read_time::date".
In the case of the latter, do you really need a coercion to date?
If it's a timestamp column, I'd think not. As for the former,
if the table doesn't get a lot of updates then creating an expression
index on that expression might be useful.
regards, tom lane
