Re: Making Vars outer-join aware

On Sat, Dec 24, 2022 at 2:20 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> I shoved some preliminary refactoring into the 0001 patch,
> notably splitting deconstruct_jointree into two passes.
> 0002-0009 cover the same ground as they did before, though
> with some differences in detail. 0010-0012 are new work
> mostly aimed at removing kluges we no longer need.

I'm looking at 0010-0012 and I really like the changes and removals
there. Thanks for the great work!

For 0010, the change seems quite independent. I think maybe we can
apply it to HEAD directly.

For 0011, I found that some clauses that were outerjoin_delayed and thus
not equivalent before might be treated as being equivalent now. For
example

explain (costs off)
select * from a left join b on a.i = b.i where coalesce(b.j, 0) = 0 and
coalesce(b.j, 0) = a.j;
QUERY PLAN
----------------------------------
Hash Right Join
Hash Cond: (b.i = a.i)
Filter: (COALESCE(b.j, 0) = 0)
-> Seq Scan on b
-> Hash
-> Seq Scan on a
Filter: (j = 0)
(7 rows)

This is different behavior from HEAD. But I think it's an improvement.

For 0012, I'm still trying to understand JoinDomain. AFAIU all EC
members of the same EC should have the same JoinDomain, because for
constants we match EC members only within the same JoinDomain, and for
Vars if they come from different join domains they will have different
nullingrels and thus will not match. So I wonder if we can have the
JoinDomain kept in EquivalenceClass rather than in each
EquivalenceMembers.

Thanks
Richard