Re: 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!