Re: Parallel Aggregate

Hi,

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
width=16)
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!

Cheers,

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(dot)rowley(at)2ndquadrant(dot)com>
wrote:

> On 12 March 2016 at 16:31, David Rowley <david(dot)rowley(at)2ndquadrant(dot)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
>
>
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>

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