Re: parallel append vs. simple UNION ALL

I have applied 0001 on pg-head, and while playing with regression tests, it crashed with below test case.

postgres=# SET min_parallel_table_scan_size=0;
SET
postgres=# SELECT * FROM information_schema.foreign_data_wrapper_options ORDER BY 1, 2, 3;
server closed the connection unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.

--logfile
2018-02-26 22:06:07.331 IST [43508] LOG:  database system is ready to accept connections
TRAP: FailedAssertion("!(subpath->parallel_safe)", File: "pathnode.c", Line: 1813)
2018-02-26 22:06:42.345 IST [43508] LOG:  server process (PID 43519) was terminated by signal 6: Aborted
2018-02-26 22:06:42.345 IST [43508] DETAIL:  Failed process was running: SELECT * FROM information_schema.foreign_data_wrapper_options ORDER BY 1, 2, 3;