CREATE SERVER fdw_explain FOREIGN DATA WRAPPER brainfuck_fdw;
-- Prints "A" and "B" on their own lines.
CREATE FOREIGN TABLE ab (line text)
    SERVER fdw_explain
    OPTIONS (program '++++++++[>++++++++<-]>+.<++++++++++.>+.<.');
SELECT set_config(name, 'on', false) FROM pg_settings
    WHERE name IN ('debug_parallel_query', 'force_parallel_mode');
 set_config 
------------
 on
(1 row)

EXPLAIN (COSTS OFF) SELECT * FROM ab;
        QUERY PLAN        
--------------------------
 Gather
   Workers Planned: 1
   Single Copy: true
   ->  Foreign Scan on ab
(4 rows)

-- The machine can't filter, so WHERE clauses stay in the plan.
EXPLAIN (COSTS OFF) SELECT * FROM ab WHERE line <> 'A';
             QUERY PLAN              
-------------------------------------
 Gather
   Workers Planned: 1
   Single Copy: true
   ->  Foreign Scan on ab
         Filter: (line <> 'A'::text)
(5 rows)

-- A table has a single `line text` column.
CREATE FOREIGN TABLE int_line (line int) SERVER fdw_explain OPTIONS (program '+');
EXPLAIN (COSTS OFF) SELECT * FROM int_line;
ERROR:  column "line" of foreign table "int_line" must be of type text
DROP SERVER fdw_explain CASCADE;
NOTICE:  drop cascades to 2 other objects
DETAIL:  drop cascades to foreign table ab
drop cascades to foreign table int_line