Oddity in EXPLAIN for foreign/custom join pushdown plans

Hi,

I noticed that currently the core doesn't show any information on the
target relations involved in a foreign/custom join in EXPLAIN, by
itself.  Here is an example:

-- join two tables
EXPLAIN (COSTS false, VERBOSE)
SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY
t1.c3, t1.c1 OFFSET 100 LIMIT 10;
                      QUERY PLAN           \


-------------------------------------------------------------------------------------------------------------------------------------------------------\
-----------------------------------
    Limit
      Output: t1.c1, t2.c1, t1.c3
      ->  Foreign Scan
            Output: t1.c1, t2.c1, t1.c3
            Relations: (public.ft1 t1) INNER JOIN (public.ft2 t2)
            Remote SQL: SELECT r1."C 1", r1.c3, r2."C 1" FROM ("S 1"."T
1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r1."C 1" = r2."C 1")))) ORDER BY
r1.c3 ASC N\
ULLS LAST, r1."C 1" ASC NULLS LAST
(6 rows)

postgres_fdw shows the target relations in the Relations line, as shown
above, but I think that the core should show such information
independently of FDWs; in the above example replace "Foreign Scan" with
"Foreign Join on public.ft1 t1, public.ft2 t2".  Please find attached a
patch for that.  Comments welcome!

Best regards,
Etsuro Fujita