[HACKERS] COPY (query) TO ... doesn't allow parallelism

Hi,

At the moment $subject doesn't allow parallelism, because copy.c's
pg_plan_query() invocation doesn't set the CURSOR_OPT_PARALLEL_OK
flag.

To me that appears to be an oversight rather than intentional.  A
somewhat annoying one at that, because it's not uncommong to use COPY to
execute reports to a CSV file and such.

Robert, am I missing a complication?

I personally think we should fix this in 9.6 and 10, but I've to admit
I'm not entirely impartial, because Citus hit this...

Greetings,

Andres Freund