Specifically, pass the outer plan's PlanState instead of our own
PlanState.  At present, ExecContextForcesOids doesn't actually care
which PlanState we pass; it just looks through to the underlying
EState to find the result relation or top-level eflags.  However, in
the future it might care.  If that happens, and if our goal is to get
a tuple descriptor that matches that of the outer plan, then I think
what we care about is whether the outer plan's context forces OIDs,
rather than whether our own context forces OIDs, just as we use the
outer node's target list rather than our own.

Patch by me, reviewed by Amit Kapila.

Discussion: http://postgr.es/m/CA+TgmoZ0ZL=cesZFq8c9NnfK6bqy-wwUd3_74iYGodYrSoQ7Fw@mail.gmail.com