Re: Parallel Queries and PostGIS

On Mon, Mar 28, 2016 at 9:45 AM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> Paul,
>
> * Paul Ramsey (pramsey(at)cleverelephant(dot)ca) wrote:
>> I spent some time over the weekend trying out the different modes of
>> parallel query (seq scan, aggregate, join) in combination with PostGIS
>> and have written up the results here:
>>
>> http://blog.cleverelephant.ca/2016/03/parallel-postgis.html
>
> Neat!
>
> Regarding aggregate parallelism and the cascaded union approach, though
> I imagine in other cases as well, it seems like having a
> "final-per-worker" function for aggregates would be useful.
>
> Without actually looking at the code at all, it seems like that wouldn't
> be terribly difficult to add.
>
> Would you agree that it'd be helpful to have for making the st_union()
> work better in parallel?

For our particular situation w/ ST_Union, yes, it would be ideal to be
able to run a worker-side combine function as well as the master-side
one. Although the cascaded union would be less effective spread out
over N nodes, doing it only once per worker, rather than every N
records would minimize the loss of effectiveness.

P