> On Jan 13, 2020, at 3:19 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > 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. >
Thanks for the suggestions. Disabling hash aggregates actually made things even worse: (https://explain.depesz.com/s/cjDg <https://explain.depesz.com/s/cjDg>), so even if that wasn’t a brute-force option, it doesn’t appear to be a good one. Creating an index on the time_bucket expression didn’t seem to make any difference, and my data does get a lot of additions (though virtually no changes) anyway (about 1 additional record per second). As far as coercion to date, that’s so I can do queries bounded by date, and actually have all results from said date included. That said, I could of course simply make sure that when I get a query parameter of, say, 2020-1-13, I expand that into a full date-time for the end of the day. However, doing so for a test query didn’t seem to make much of a difference either: https://explain.depesz.com/s/X5VT <https://explain.depesz.com/s/X5VT> So, to summarise: Set enable_hasagg=off: worse Index on time_bucket expression: no change in execution time or query plan that I can see Get rid of coercion to date: *slight* improvement. 14.692 seconds instead of 15.5 seconds. And it looks like the row count estimates were actually worse. Lower work_mem, forcing a disk sort and completely different query plan: Way, way better (around 6 seconds) …so so far, it looks like the best option is to lower the work_mem, run the query, then set it back? --- Israel Brewster Software Engineer Alaska Volcano Observatory Geophysical Institute - UAF 2156 Koyukuk Drive Fairbanks AK 99775-7320 Work: 907-474-5172 cell: 907-328-9145 > regards, tom lane
