Re: Making Vars outer-join aware

On Sun, Nov 6, 2022 at 5:53 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> I wrote:
> > I've been working away at this patch series, and here is an up-to-date
> > version.
>
> This needs a rebase after ff8fa0bf7 and b0b72c64a. I also re-ordered
> the patches so that the commit messages' claims about when regression
> tests start to pass are true again. No interesting changes, though.

I'm reviewing the part about multiple version clauses, and I find a case
that may not work as expected. I tried with some query as below

(A leftjoin (B leftjoin C on (Pbc)) on (Pab)) left join D on (Pcd)

Assume Pbc is strict for B and Pcd is strict for C.

According to identity 3, we know one of its equivalent form is

((A leftjoin B on (Pab)) leftjoin C on (Pbc)) left join D on (Pcd)

For outer join clause Pcd, we would generate two versions from the first
form

Version 1: C Vars with nullingrels as {A/B}
Version 2: C Vars with nullingrels as {B/C, A/B}

I understand version 2 is reasonable as the nullingrels from parser
would be set as that. But it seems version 1 is not applicable in
either form.

Looking at the two forms again, it seems the expected two versions for
Pcd should be

Version 1: C Vars with nullingrels as {B/C}
Version 2: C Vars with nullingrels as {B/C, A/B}

With this we may have another problem that the two versions are both
applicable at the C/D join according to clause_is_computable_at(), in
both forms.

Another thing is I believe we have another equivalent form as

(A left join B on (Pab)) left join (C left join D on (Pcd)) on (Pbc)

Currently this form cannot be generated because of the issue discussed
in [1]. But someday when we can do that, I think we should have a third
version for Pcd

Version 3: C Vars with empty nullingrels

[1]
https://www.postgresql.org/message-id/flat/CAMbWs4_8n5ANh_aX2PinRZ9V9mtBguhnRd4DOVt9msPgHmEMOQ%40mail.gmail.com

Thanks
Richard