On Tuesday, 15 March 2016, Robert Haas <
robertmhaas@gmail.com> wrote:
> Does the cost of the aggregate function come into this calculation at
> all? In PostGIS land, much smaller numbers of rows can generate loads
> that would be effective to parallelize (worker time much >> than
> startup cost).
Unfortunately, no - only the table size. This is a problem, and needs
to be fixed. However, it's probably not going to get fixed for 9.6.
:-(
Any chance of getting a GUC (say min_parallel_degree) added to allow setting the initial value of parallel_degree, then changing the small relation check to also pass if parallel_degree > 1?
That way you could set min_parallel_degree on a query by query basis if you are running aggregates which you know will take a lot of CPU.
I suppose it wouldn't make much sense at all to set globally though, so it could just confuse matters.
Cheers,
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