Re: Aggregate and many LEFT JOIN

I have better results with this version. Basically, I run a first query
only made for aggregation, and then do a JOIN to get other needed data.

* SQL : http://paste.debian.net/1070007/
* EXPLAIN: https://explain.depesz.com/s/D0l

Not really "fast", but I gained 30%

Le lun. 25 févr. 2019 à 09:54, kimaidou <kimaidou(at)gmail(dot)com> a écrit :

> Thanks for your answers. I tried with
> > set session work_mem='250MB';
> > set session geqo_threshold = 20;
> > set session join_collapse_limit = 20;
>
> It seems to have no real impact :
> https://explain.depesz.com/s/CBVd
>
> Indeed an index cannot really be used for sorting here, based on the
> complexity of the returned fields.
> Wich strikes me is that if I try to simplify it a lot, removing all data
> but the main table (occtax.observation) primary key cd_nom and aggregate,
> the query plan should be able tu use the cd_nom index for sorting and
> provide better query plan (hash aggregate), but it does not seems so :
>
> * SQL ; http://paste.debian.net/hidden/c3ee7889/
> * EXPLAIN : https://explain.depesz.com/s/FR3h -> a group aggregate is
> used, which : GroupAggregate 1 10,639.313 ms 72.6 %
>
> It is better, but I think 10s for such a query seems bad perf for me.
>
> Regards
> Michaël
>
> Le ven. 22 févr. 2019 à 19:06, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> a écrit :
>
>> Michael Lewis <mlewis(at)entrata(dot)com> writes:
>> > Does the plan change significantly with this-
>> > set session work_mem='250MB';
>> > set session geqo_threshold = 20;
>> > set session join_collapse_limit = 20;
>>
>> Yeah ... by my count there are 16 tables in this query, so raising
>> join_collapse_limit to 15 is not enough to ensure that the planner
>> considers all join orders. Whether use of GEQO is a big problem
>> is harder to say, but it might be.
>>
>> regards, tom lane
>>
>