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

Tomasz Rybak <tomasz(dot)rybak(at)post(dot)pl> 于2024年5月31日周五 04:21写道：

> 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.
>

Thanks for reviewing this patch.

> 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
>

I added a test case that inner rel is not parallel safe. Actually, matpath
will not create
if inner rel is not parallel safe. So I didn't add test case for the
second scenario.

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.
>

Yeah, Richard commented the v1 patch about JOIN_UNIQUE_INNER in [1]

* I think we should not consider materializing the cheapest inner path
if we're doing JOIN_UNIQUE_INNER, because in this case we have to
unique-ify the inner path.

We don't consider material inner path if jointype is JOIN_UNIQUE_INNER in
match_unsorted_order().
So here is as same logic as match_unsorted_order(). I added comments to
explain why.

[1]
https://www.postgresql.org/message-id/CAMbWs49LbQF_Z0iKPRPnTHfsRECT7M-4rF6ft5vpW1ARSpBkPA%40mail.gmail.com

--
Tender Wang
OpenPie: https://en.openpie.com/