Function outer_join_nullable_qualifiers 

pub fn outer_join_nullable_qualifiers(from: &SourcePlan) -> BTreeSet<String>

Qualifiers whose rows can be synthesized as NULLs by an outer join cannot receive an arbitrary WHERE predicate before that join. A predicate such as right.id IS NULL accepts the synthesized row; pushing it into the right scan first can remove a real match, manufacture a NULL-extended row, and turn a non-result into a result. Keep predicates on these qualifiers above the outer join unless a separate rewrite has first reduced it to an inner join.