Re: parallel.c is not marked as test covered

On Tue, Jun 21, 2016 at 1:24 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Mon, Jun 20, 2016 at 1:13 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Robert Haas <robertmhaas@gmail.com> writes:
> >> On Sun, Jun 19, 2016 at 10:23 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >>> Personally, I'm +1 for such tinkering if it makes the feature either more
> >>> controllable or more understandable.  After reading the comments at the
> >>> head of nodeGather.c, though, I don't think that single_copy is either
> >>> understandable or useful, and merely renaming it won't help.  Apparently,
> >>> it runs code in the worker, except when it doesn't, and even when it does,
> >>> it's absolutely guaranteed to be a performance loss because the leader is
> >>> doing nothing.  What in the world is the point?
> >
> >> The single_copy flag allows a Gather node to have a child plan which
> >> is not intrinsically parallel.
> >
> > OK, but I'm very dubious of your claim that this has any use except for
> > testing purposes.  It certainly has no such use today.  Therefore, the
> > behavior of falling back to running in the leader seems like an
> > anti-feature to me.  If we want to test running in a worker, then we
> > want to test that, not maybe test it.
> >
> > I propose that the behavior we actually want here is to execute in
> > a worker, full stop.  If we can't get one, wait till we can.  If we
> > can't get one because no slots have been allocated at all, fail.
> > That would make the behavior deterministic enough for the regression
> > tests to rely on it.
>

+1.

> I agree that for force_parallel_mode testing, that behavior would be useful.
>
> I am also pretty confident we're going to want the behavior where the
> leader runs the plan if, and only if, no workers can be obtained for
> other purposes.  However, I agree that's not essential today.
>
> > And while I don't know what this mode should be called, I'm pretty sure
> > that neither "single_copy" nor "pipeline" are useful descriptions.
>
> Maybe we should make this an enum rather than a boolean, since there
> seem to be more than two useful behaviors.
>