A GUC to prevent leader processes from running subplans?

Hi hackers,

While testing parallelism work I've wanted to be able to prevent
gather nodes from running the plan in the leader process, and I've
heard others say the same.  One way would be to add a GUC
"multiplex_gather", like in the attached patch.  If you set it to off,
Gather and Gather Merge won't run the subplan unless they have to
because no workers could be launched.  I thought about adding a new
value for force_parallel_mode instead, but someone mentioned they
might want to do this on a production system too and
force_parallel_mode is not really for end users.  Better ideas?

-- 
Thomas Munro
http://www.enterprisedb.com