Re: Parallel Aggregate

On Mon, Mar 14, 2016 at 3:56 PM, Paul Ramsey <pramsey(at)cleverelephant(dot)ca> wrote:
> On Sun, Mar 13, 2016 at 7:31 PM, David Rowley
> <david(dot)rowley(at)2ndquadrant(dot)com> wrote:
>> 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.
>
> 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.
:-(

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company