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. > >> 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 >
