My attempts to test this all got stuck in wait_on_slots(). I haven't looked too closely, but I suspect the issue is that the socket never becomes readable because we don't send a query. If I set free_slot->inUse to false before printing the command, it no longer hangs. We probably want to create a function in parallel_slot.c to mark slots that we don't intend to give a query as idle.
Would that be preferable to skipping the creation of extra connections for parallel workers? I can see it both ways. On the one hand we want to give as true a reflection of "what would happen with these options", and on the other hand one could view the creation of extra workers as "real" vs a dry run.
