Re: WIP: Upper planner pathification

On Sun, Mar 6, 2016 at 9:59 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Perhaps it was intentional when written, but if Robert's advice is correct
> that the new upper-planner path nodes should copy up parallel_degree from
> their children, then it cannot be the case that parallel_degree>0 in a
> node above the scan level implies that that node type has any special
> behavior for parallelism.
>
> I continue to bemoan the lack of documentation about what these fields
> mean.  As far as I can find, the sum total of the documentation about
> this field is
>
>     int         parallel_degree; /* desired parallel degree; 0 = not parallel */

While it doesn't particularly relate to parallel joins, I've expressed
a general concern about the max_parallel_degree GUC that I think is
worth considering again:

http://www.postgresql.org/message-id/CAM3SWZRs1mTvrKkAsY1XBShGZXkD6-HNxX3gq7x-p-dz0ZtkMg@mail.gmail.com

In summary, I think it's surprising that a max_parallel_degree of 1
doesn't disable parallel workers entirely.

-- 
Peter Geoghegan