Before flattening procedure we just look through the quals of subquery, pull to the upper level OpExpr's containing variables from the upper relation and replace their positions in the quals with true expression. Further, the flattening machinery works as usual.
Hmm, I'm not sure this patch works correctly in all cases. It seems to me this patch pulls up the subquery without checking the constraints imposed by lateral references. If its quals contain any lateral references to rels outside a higher outer join, we would need to postpone quals from below an outer join to above it, which is probably incorrect. As an example, consider
select * from a left join b on b.i in (select c.i from c where c.j = a.j);
If we pull up the ANY SubLink into parent query and pull up its qual into upper level, as what the patch does, then its qual 'c.j = a.j' would have to be postponed past the B/C semi join, which is totally wrong. Doing this would firstly trigger the assertion failure in distribute_qual_to_rels
Even if we ignore these assertion checks, in the final plan we would have to access the RHS of the B/C semi join, i.e. C, to evaluate qual 'c.j = a.j' at the join level of A/BC join, which is wrong.
