On Tuesday, 15 March 2016, Robert Haas <robertmh...@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.



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