Re: Choosing parallel_degree

On Mon, Mar 14, 2016 at 9:25 PM, David Rowley
<david(dot)rowley(at)2ndquadrant(dot)com> wrote:
> Over in [1] James mentioned about wanting more to be able to have more
> influence over the partial path's parallel_degree decision. At risk
> of a discussion on that hijacking the parallel aggregate thread, I
> thought I'd start this for anyone who would want to discuss making
> changes to that.
>
> I've attached a simple C program which shows the parallel_degree which
> will be chosen at the moment. For now it's based on the size of the
> base relation. Perhaps that will need to be rethought later, perhaps
> based on costs. But I just don't think it's something for 9.6.

I thought about this a bit more. There are a couple of easy things we
could do here.

The 1000-page threshold could be made into a GUC.

We could add a per-table reloption for parallel-degree that would
override the calculation.

Neither of those things is very smart, but they'd probably both help
some people. If someone is able to produce a patch for either or both
of these things *quickly*, we could possibly try to squeeze it into
9.6 as a cleanup of work already done.

--
Robert Haas
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