Re: foreign join error "variable not found in subplan target list"

* I think we should avoid modifying the pathtarget, because it would
be the default pathtarget, which other paths might reference.  I think
it’s safe to use a copied pathtarget, like the attached.

* I think this issue occurs also when there are PlaceHolderVars in the
relation’s reltarget.  Here is an example:

EXPLAIN (VERBOSE, COSTS OFF)
SELECT * FROM local_tbl LEFT JOIN (SELECT ft1.*, COALESCE(ft1.c3 ||
ft2.c3, 'foobar') FROM ft1 INNER JOIN ft2 ON (ft1.c1 = ft2.c1 AND
ft1.c1 < 100)) ss ON (local_tbl.c1 = ss.c1) ORDER BY local_tbl.c1 FOR
UPDATE OF local_tbl;
ERROR:  variable not found in subplan target list

where local_tbl, ft1, and ft2 are local/foreign tables defined as in
postgres_fdw.sql.  To fix, I modified the patch so that we add to the
pathtarget not only columns required for evaluating the local
conditions but columns required for evaluating the PlaceHoderVars.

* The test case reported in this thread produces the planner error
only in v14 and later, but I think this issue exists since v9.6.  So I
created/added new test cases, including the above one, that would
otherwise produce the error even in previous versions.