Re: Parallel Aggregate

On 14 March 2016 at 17:05, James Sewell <james.sewell@lisasoft.com> wrote:
>
> Hi again,
>
> I've been playing around with inheritance combined with this patch. Currently it looks like you are taking
max(parallel_degree)from all the child tables and using that for the number of workers.
 
>
> For large machines it makes much more sense to use sum(parallel_degree) - but I've just seen this comment in the
code:
>
>     /*
>      * Decide what parallel degree to request for this append path.  For
>      * now, we just use the maximum parallel degree of any member.  It
>      * might be useful to use a higher number if the Append node were
>      * smart enough to spread out the workers, but it currently isn't.
>      */
>
> Does this mean that even though we are aggregating in parallel, we are only operating on one child table at a time
currently?

There is nothing in the planner yet, or any patch that I know of to
push the Partial Aggregate node to below an Append node. That will
most likely come in 9.7.

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