| From: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> |
|---|---|
| To: | James Sewell <james(dot)sewell(at)lisasoft(dot)com> |
| Cc: | "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Parallel Aggregate |
| Date: | 2016-03-14 02:31:01 |
| Message-ID: | CAKJS1f-RaS=iNCOML_TDo1qF5V=EDPD0LR1tXWCAsu4Wgv0MYw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On 14 March 2016 at 14:52, James Sewell <james(dot)sewell(at)lisasoft(dot)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
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