FULL JOIN planner deficiency

Consider this simple query:

regression=# explain select * from
int8_tbl as a1 full join (select 1 as id) as a2 on (a1.q1 = a2.id);
QUERY PLAN
------------------------------------------------------------------
Hash Full Join (cost=0.03..1.11 rows=5 width=20)
Hash Cond: (a1.q1 = (1))
-> Seq Scan on int8_tbl a1 (cost=0.00..1.05 rows=5 width=16)
-> Hash (cost=0.02..0.02 rows=1 width=4)
-> Result (cost=0.00..0.01 rows=1 width=4)
(5 rows)

Not too exciting-looking. But this ought to be exactly equivalent:

regression=# create table dual();
CREATE TABLE
regression=# insert into dual default values;
INSERT 0 1
regression=# explain select * from
int8_tbl as a1 full join (select 1 as id from dual) as a2 on (a1.q1 = a2.id);
ERROR: FULL JOIN is only supported with merge-joinable or hash-joinable join conditions

I ran into this while testing the patch mentioned in
<5395(dot)1539275668(at)sss(dot)pgh(dot)pa(dot)us>, which basically causes the FROM-less
subselect to be treated the same as the "FROM dual" case. But it's
a pre-existing, and long-standing, problem.

The root of the problem is that once the constant "1" has been pulled
up from the sub-select, we have a join qual that looks like "a1.q1 = 1",
and that is not a mergeable or hashable join qual, because it fails to
compare expressions from the two sides of the join.

I spent awhile thinking about whether we could generalize our notion
of mergeability, or hashability, to make this work, but it soon made
my head hurt. Even if it's possible it would likely not be a change
we'd want to backpatch.

However, there's another way to deal with it, which is to wrap the
pulled-up constant in a PlaceHolderVar, which will cause it to act
like a Var for the purpose of recognizing a qual as mergeable/hashable.
The attached two-line (sans tests) patch does this and fixes the problem.

While this could in theory reduce our ability to optimize things
(by making expressions look unequal that formerly looked equal),
I do not think it's a big problem because our ability to optimize
full joins is pretty darn limited anyway.

Given the lack of complaints, I'm not real sure whether this is
worth back-patching. Thoughts?

regards, tom lane