join plan with unexpected var clauses

Hi,

At a customer we came across a curious plan (see attached testcase).

Given the testcase we see that the outer semi join tries to join the 
outer with the inner table id columns, even though the middle table id 
column is also there. Is this expected behavior?

The reason i'm asking is two-fold:
- the inner hash table now is bigger than i'd expect and has columns 
that you would normally not select on.
- the middle join now projects the inner as result, which is quite 
suprising and seems invalid from a SQL standpoint.

Plan:
  Finalize Aggregate
    Output: count(*)
    ->  Gather
          Output: (PARTIAL count(*))
          Workers Planned: 4
          ->  Partial Aggregate
                Output: PARTIAL count(*)
                ->  Parallel Hash Semi Join
                      Hash Cond: (_outer.id3 = _inner.id2)
                      ->  Parallel Seq Scan on public._outer
                            Output: _outer.id3, _outer.extra1
                      ->  Parallel Hash
                            Output: middle.id1, _inner.id2
                            ->  Parallel Hash Semi Join
                                  Output: middle.id1, _inner.id2
                                  Hash Cond: (middle.id1 = _inner.id2)
                                  ->  Parallel Seq Scan on public.middle
                                        Output: middle.id1
                                  ->  Parallel Hash
                                        Output: _inner.id2
                                        ->  Parallel Seq Scan on 
public._inner
                                              Output: _inner.id2

Kind regards,
Luc
Swarm64