Re: Increasing work_mem slows down query, why?

po 30. 3. 2020 v 18:36 odesílatel Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> napsal:

> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> > CTE scan has only 1100 rows, public.rhnpackagecapability has 490964
> rows.
> > But planner does hash from public.rhnpackagecapability table. It cannot
> be
> > very effective.
>
> [ shrug... ] Without stats on the CTE output, the planner is very
> leery of putting it on the inside of a hash join. The CTE might
> produce output that ends up in just a few hash buckets, degrading
> the join to something not much better than a nested loop. As long
> as there's enough memory to hash the known-well-distributed table,
> putting it on the inside is safer and no costlier.
>

ok

Regards

Pavel

> regards, tom lane
>