Re: Worse performance with higher work_mem?

Israel Brewster <ijbrewster@alaska.edu> writes:
> In looking at the explain analyze output, I noticed that it had an “external merge Disk” sort going on, accounting
forabout 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_memparameter. Whereupon the query plan got much simpler, and performance of said query completely tanked,
increasingto about 15.5 seconds runtime (https://explain.depesz.com/s/Kl0S <https://explain.depesz.com/s/Kl0S>), most
ofwhich 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