I've done some testing with one of my data sets in an 8VPU virtual
environment and this is looking really, really good.

My test query is:

SELECT pageview, sum(pageview_count)
FROM fact_agg_2015_12
GROUP BY date_trunc('DAY'::text, pageview);

The query returns 15 rows. The fact_agg table is 5398MB and holds around 25
million records.

Explain with a max_parallel_degree of 8 tells me that the query will only
use 6 background workers. I have no indexes on the table currently.

Finalize HashAggregate  (cost=810142.42..810882.62 rows=59216 width=16)
   Group Key: (date_trunc('DAY'::text, pageview))
   ->  Gather  (cost=765878.46..808069.86 rows=414512 width=16)
         Number of Workers: 6
         ->  Partial HashAggregate  (cost=764878.46..765618.66 rows=59216
               Group Key: date_trunc('DAY'::text, pageview)
               ->  Parallel Seq Scan on fact_agg_2015_12
 (cost=0.00..743769.76 rows=4221741 width=12)

I am getting the following timings (everything was cached before I started
tested). I didn't average the runtime, but I ran each one three times and
took the middle value.

*max_parallel_degree     runtime*
0                                          11693.537 ms
1                                          6387.937 ms
2                                         4328.629 ms
3                                         3292.376 ms
4                                         2743.148 ms
5                                         2278.449 ms
6                                         2000.599 ms

I'm pretty happy!


James Sewell,
PostgreSQL Team Lead / 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 Mon, Mar 14, 2016 at 8:44 AM, David Rowley <david.row...@2ndquadrant.com>

> On 12 March 2016 at 16:31, David Rowley <david.row...@2ndquadrant.com>
> wrote:
> > I've attached an updated patch which is based on commit 7087166,
> > things are really changing fast in the grouping path area at the
> > moment, but hopefully the dust is starting to settle now.
> The attached patch fixes a harmless compiler warning about a possible
> uninitialised variable.
> --
>  David Rowley                   http://www.2ndQuadrant.com/
>  PostgreSQL Development, 24x7 Support, Training & Services
> --
> Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
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