Re: A GUC to prevent leader processes from running subplans?

On Sat, Oct 21, 2017 at 8:09 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Oct 17, 2017 at 7:27 AM, Thomas Munro
> <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
>> 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?
>
> I don't think overloading force_parallel_mode is a good idea, but
> having some other GUC for this seems OK to me. Not sure I like
> multiplex_gather, though.

How about parallel_leader_participation = on|off? The attached
version has it that way, and adds regression tests to exercise on, off
and off-but-couldn't-start-any-workers for both kinds of gather node.

I'm not sure why node->need_to_rescan is initialised by both
ExecGatherInit() and ExecGather(). Only the latter's value matters,
right?

I've added this to the January Commitfest.

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