Re: Parallel Aggregate

Hi again,

This is probably me missing something, but is there a reason parallel
aggregate doesn't seem to ever create append nodes containing Index scans?

SET random_page_cost TO 0.2;
SET max_parallel_degree TO 8;

postgres=# explain SELECT sum(count_i) FROM base GROUP BY view_time_day;
QUERY PLAN
-------------------------------------------------------------------------------------------------
Finalize GroupAggregate (cost=310596.32..310598.03 rows=31 width=16)
Group Key: view_time_day
-> Sort (cost=310596.32..310596.79 rows=186 width=16)
Sort Key: view_time_day
-> Gather (cost=310589.00..310589.31 rows=186 width=16)
Number of Workers: 5
-> Partial HashAggregate (cost=310589.00..310589.31
rows=31 width=16)
Group Key: view_time_day
-> Parallel Seq Scan on base (cost=0.00..280589.00
rows=6000000 width=12)

SET max_parallel_degree TO 0;

postgres=# explain SELECT sum(count_i) FROM base GROUP BY view_time_day;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------
GroupAggregate (cost=0.56..600085.92 rows=31 width=16)
Group Key: view_time_day
-> Index Only Scan using base_view_time_day_count_i_idx on base
(cost=0.56..450085.61 rows=30000000 width=12)
(3 rows)

Cheers,

James Sewell,
Solutions Architect
______________________________________

Level 2, 50 Queen St, Melbourne VIC 3000

*P *(+61) 3 8370 8000 *W* www.lisasoft.com *F *(+61) 3 8370 8099

On Thu, Mar 17, 2016 at 8:08 AM, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
wrote:

> On 17 March 2016 at 01:29, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > On Wed, Mar 16, 2016 at 8:19 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
> >> Isn't it better to call it as Parallel Aggregate instead of Partial
> >> Aggregate. Initialy, we have kept Partial for seqscan, but later on we
> >> changed to Parallel Seq Scan, so I am not able to think why it is
> better to
> >> call Partial incase of Aggregates.
> >
> > I think partial is the right terminology. Unlike a parallel
> > sequential scan, a partial aggregate isn't parallel-aware and could be
> > used in contexts having nothing to do with parallelism. It's just
> > that it outputs transition values instead of a finalized value.
>
> +1 the reason the partial aggregate patches have been kept separate
> from the parallel aggregate patches is that partial aggregate will
> serve for many other purposes. Parallel Aggregate is just one of many
> possible use cases for this, so it makes little sense to give it a
> name according to a single use case.
>
> --
> David Rowley http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services
>
>
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