Re: Parallel Aggregate - Mailing list pgsql-hackers
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
______________________________________
On Thu, Mar 17, 2016 at 8:08 AM, David Rowley <david.rowley@2ndquadrant.com> wrote:
On 17 March 2016 at 01:29, Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Mar 16, 2016 at 8:19 AM, Amit Kapila <amit.kapila16@gmail.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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