Re: Improve planner cost estimations for alternative subplans

On Fri, Jun 5, 2020 at 9:08 AM Alexey Bashtanov <bashtanov(at)imap(dot)cc> wrote:

>
> In [1] we found a situation where it leads to a suboptimal plan,
> as it bloats the overall cost into large figures,
> a decision related to an outer part of the plan look negligible to the
> planner,
> and as a result it doesn't elaborate on choosing the optimal one.
>
>
I've just started looking at this patch today, but I was wondering if
you might include a test case which minimally reproduces the original
problem you had.
The only plan diff I see is in updatable_views.sql, and it doesn't
illustrate the problem as well as a more straightforward SELECT query
with EXISTS sublink might.

After putting in some logging, I see that there are only a
few non-catalog queries which exercise this code path.
This query from groupingsets.sql is an example of one such query:

select ten, sum(distinct four) from onek a
group by grouping sets((ten,four),(ten))
having exists (select 1 from onek b where sum(distinct a.four) = b.four);

But, the chosen plan for this query stays the same.

It would be helpful to see a query where a different plan is chosen
because of this change that is not from updatable_views.sql.

--
Melanie Plageman