Re: query performance issue

2017-11-15 20:58 GMT+01:00 Gunther <raj(at)gusw(dot)net>:

>
> On 11/15/2017 8:12, Pavel Stehule wrote:
>
> There is wrong plan due wrong estimation
>
> for this query you should to penalize nested loop
>
> set enable_nestloop to off;
>
> before evaluation of this query
>
>
> You are not the only one with this issue. May I suggest to look at this
> thread a little earlier this month.
>
> http://www.postgresql-archive.org/OLAP-reporting-queries-
> fall-into-nested-loops-over-seq-scans-or-other-horrible-
> planner-choices-tp5990160.html
>
> where this has been discussed in some length.
>

It is typical issue. The source of these problems are correlations between
columns (it can be fixed partially by multicolumn statistics in PostgreSQL
10). Another problem is missing multi table statistics - PostgreSQL planner
expects so any value from dictionary has same probability, what is not
usually true. Some OLAP techniques like calendar tables has usually very
bad impact on estimations with this results.

Regards

Pavel

> regards,
> -Gunther
>
>
>