Re: Problem about postponing gathering partial paths for topmost scan/join rel

Richard Guo <guofenglinux(at)gmail(dot)com> wrote:

> On Fri, Jul 15, 2022 at 5:00 PM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
>
> On Fri, Jul 15, 2022 at 4:03 PM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
>
> On Thu, Jul 14, 2022 at 10:02 PM Antonin Houska <ah(at)cybertec(dot)at> wrote:
>
> I'd prefer a test that demonstrates that the Gather node at the top of the
> "subproblem plan" is useful purely from the *cost* perspective, rather than
> due to executor limitation.
>
> This patch provides an additional path (Gather atop of subproblem) which
> was not available before. But your concern makes sense that we need to
> show this new path is valuable from competing on cost with other paths.
>
> How about we change to Nested Loop at the topmost? Something like:
>
> Maybe a better example is that we use a small table 'c' to avoid the
> Gather node above scanning 'c', so that the path of parallel nestloop is
> possible to be generated.
>
> Update the patch with the new test case.

ok, this makes sense to me. Just one minor suggestion: the command

alter table d_star reset (parallel_workers);

is not necessary because it's immediately followed by

rollback;

I'm going to set the CF entry to "ready for committer'".

--
Antonin Houska
Web: https://www.cybertec-postgresql.com