Re: Making Vars outer-join aware

 

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