On Tue, Oct 20, 2020 at 10:50 AM Pavel Stehule <pavel.steh...@gmail.com> wrote:
> > > út 20. 10. 2020 v 11:59 odesílatel Mats Julian Olsen < > m...@duneanalytics.com> napsal: > >> On Tue, Oct 20, 2020 at 9:50 AM David Rowley <dgrowle...@gmail.com> >> wrote: >> >>> On Tue, 20 Oct 2020 at 22:38, Mats Julian Olsen <m...@duneanalytics.com> >>> wrote: >>> > >>> > The crux of our issue is that the query planner chooses a nested loop >>> join for this query. Essentially making this query (and other queries) take >>> a very long time to complete. In contrast, by toggling `enable_nestloop` >>> and `enable_seqscan` off we can take the total runtime down from 16 minutes >>> to 2 minutes. >>> > >>> > 1) Vanilla plan (16 min) : https://explain.depesz.com/s/NvDR >>> > 2) enable_nestloop=off (4 min): https://explain.depesz.com/s/buKK >>> > 3) enable_nestloop=off; enable_seqscan=off (2 min): >>> https://explain.depesz.com/s/0WXx >>> > >>> > How can I get Postgres not to loop over 12M rows? >>> >>> You'll likely want to look at what random_page_cost is set to. If the >>> planner is preferring nested loops then it may be too low. You'll >>> also want to see if effective_cache_size is set to something >>> realistic. Higher values of that will prefer nested loops like this. >>> >> >> random_page_cost is 1.1 and effective_cache_size is '60GB' (listed in the >> gist). random_page_cost may be too low? >> > > random_page_cost 2 is safer - the value 1.5 is a little bit aggressive for > me. > Thanks Pavel. I tried changing random_page_cost from 1.1 to 2, to 3... all the way up to 10. All values resulted in the same query plan, except for 10, which then executed a parallel hash join (however with sequential scans) https://explain.depesz.com/s/Srcb. 10 seems like a way too high value for random_page_cost though? > >> >>> You may also want to reduce max_parallel_workers_per_gather. It looks >>> like you're not getting your parallel workers as often as you'd like. >>> If the planner chooses a plan thinking it's going to get some workers >>> and gets none, then that plan may be inferior the one that the planner >>> would have chosen if it had known the workers would be unavailable. >>> >> >> Interesting, here are the values for those: >> max_parallel_workers = 8 >> max_parallel_workers_per_gather = 4 >> >> >>> >>> > Let me know if there is anything I left out here that would be useful >>> for further debugging. >>> >>> select name,setting from pg_Settings where category like 'Query >>> Tuning%' and source <> 'default'; >>> select version(); >>> >> >> default_statistics_target = 500 >> effective_cache_size = 7864320 >> random_page_cost = 1.1 >> >> PostgreSQL 12.2 (Ubuntu 12.2-2.pgdg19.10+1) on x86_64-pc-linux-gnu, >> compiled by gcc (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008, 64-bit >> >>> >>> would be useful. >>> >>> David >>> >> >> Thanks David, see above for more information. >> >> -- >> Mats >> CTO @ Dune Analytics >> We're hiring: https://careers.duneanalytics.com >> > -- Mats CTO @ Dune Analytics We're hiring: https://careers.duneanalytics.com
