ISTM that for outer join identity 3, if we are given form (A leftjoin B on (Pab)) leftjoin C on (Pbc) then references to C Vars in higher qual levels would be marked with the B/C join. If we've transformed it to form A leftjoin (B leftjoin C on (Pbc)) on (Pab) then references to C Vars in higher qual levels should be adjusted to include both B/C join and A/B join in their varnullingrels.
A quick hack that comes to my mind is that for a pushed down clause we check all outer join relids it mentions and add the outer joins' commute_below to the clause's required_relids, so that after we've commuted the outer joins, the clause would still be placed in the right place.
I've realized this hack is not correct :-(. References to A Vars and B Vars in higher qual levels do not need this adjustment. So this code change would add unnecessary relids for clauses that involve A Vars or B Vars but not C Vars, resulting them being placed at higher place than needed.
Maybe what we need is to add in commute_below_l (assuming it has been recorded in SpecialJoinInfo) rather than commute_below of outer joins mentioned within the clause?
