Re: Parallel Aggregate

On 14 March 2016 at 14:52, James Sewell <james.sewell@lisasoft.com> wrote:
> One question - how is the upper limit of workers chosen?

See create_parallel_paths() in allpaths.c. Basically the bigger the
relation (in pages) the more workers will be allocated, up until
max_parallel_degree.

There is also a comment in that function which states:
/*
* Limit the degree of parallelism logarithmically based on the size of the
* relation.  This probably needs to be a good deal more sophisticated, but we
* need something here for now.
*/

So this will likely see some revision at some point, after 9.6.

-- David Rowley                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services