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.
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.
--
Robert Haas
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