Re: Making Vars outer-join aware

I wrote:
> Hans Buschmann <buschmann@nidsa.net> writes:
>> I just noticed your new efforts in this area.
>> I wanted to recurr to my old thread [1] considering constant propagation of quals.
>> [1]  https://www.postgresql.org/message-id/1571413123735.26467@nidsa.net

> Yeah, this patch series is not yet quite up to the point of improving
> that.  That area is indeed the very next thing I want to work on, and
> I did spend some effort on it last month, but I ran out of time to get
> it working.  Maybe we'll have something there for v17.

BTW, to clarify what's going on there: what I want to do is allow
the regular equivalence-class machinery to handle deductions from
equality operators appearing in LEFT JOIN ON clauses (maybe full
joins too, but I'd be satisfied if it works for one-sided outer
joins).  I'd originally hoped that distinguishing pre-nulled from
post-nulled variables would be enough to make that safe, but it's
not.  Here's an example:

        select ... from t1 left join t2 on (t1.x = t2.y and t1.x = 1);

If we turn the generic equivclass.c logic loose on these clauses,
it will deduce t2.y = 1, which is good, and then apply t2.y = 1 at
the scan of t2, which is even better (since we might be able to turn
that into an indexscan qual).  However, it will also try to apply
t1.x = 1 at the scan of t1, and that's just wrong, because that
will eliminate t1 rows that should come through with null extension.