Re: Should consider materializing the cheapest inner path in consider_parallel_nestloop()

On Tue, 2024-04-23 at 16:59 +0800, Tender Wang wrote:
>
[ cut ]
>
> I have rebased master and fixed a plan diff case.

We (me, Paul Jungwirth, and Yuki Fujii) reviewed this patch
at PgConf.dev Patch Review Workshop.
Here are our findings.

Patch tries to allow for using materialization together
with parallel subqueries.
It applies cleanly on 8fea1bd5411b793697a4c9087c403887e050c4ac
(current HEAD).
Tests pass locally on macOS and Linux in VM under Windows.
Tests are also green in cfbot (for last 2 weeks; they were
red previously, probably because of need to rebase).

Please add more tests. Especially please add some negative tests;
current patch checks that it is safe to apply materialization. It would
be helpful to add tests checking that materialization is not applied
in both checked cases:
1. when inner join path is not parallel safe
2. when matpath is not parallel safe

This patch tries to apply materialization only when join type
is not JOIN_UNIQUE_INNER. Comment mentions this, but does not
explain why. So please either add comment describing reason for that
or try enabling materialization in such a case.

Best regards.

--
Tomasz Rybak, Debian Developer <serpent(at)debian(dot)org>
GPG: A565 CE64 F866 A258 4DDC F9C7 ECB7 3E37 E887 AA8C