Re: Query Performance / Planner estimate off

On Tue, Oct 20, 2020 at 9:50 AM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> On Tue, 20 Oct 2020 at 22:38, Mats Julian Olsen <mats(at)duneanalytics(dot)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?

> 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
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