Re: Query Performance / Planner estimate off

út 20. 10. 2020 v 13:09 odesílatel Mats Julian Olsen <mats(at)duneanalytics(dot)com>
napsal:

>
>
> On Tue, Oct 20, 2020 at 10:50 AM Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
> wrote:
>
>>
>>
>> út 20. 10. 2020 v 11:59 odesílatel Mats Julian Olsen <
>> mats(at)duneanalytics(dot)com> napsal:
>>
>>> 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?
>>>
>>
>> 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?
>

it is not usual, but I know about analytics cases where is this value. But
maybe effective_cache_size is too high.

>
>
>>
>>>
>>>> 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
>