AFAICS, the Vars forced nonnullable by given clause are only used to
check if we can reduce JOIN_LEFT to JOIN_ANTI, and it is checking the
join's own quals there. It seems to me we do not need to pass down
nonnullable_vars by upper quals to the children of a join.
Attached is a patch to remove the pass-down of nonnullable_vars.
Thanks
Richard