Re: Parallel Aggregate

Hi,

Happy to test, really looking forward to seeing this stuff in core.

The explain analyze is below:

Finalize HashAggregate (cost=810142.42..810882.62 rows=59216 width=16)
(actual time=2282.092..2282.202 rows=15 loops=1)
Group Key: (date_trunc('DAY'::text, pageview_start_tstamp))
-> Gather (cost=765878.46..808069.86 rows=414512 width=16) (actual
time=2281.749..2282.060 rows=105 loops=1)
Number of Workers: 6
-> Partial HashAggregate (cost=764878.46..765618.66 rows=59216
width=16) (actual time=2276.879..2277.030 rows=15 loops=7)
Group Key: date_trunc('DAY'::text, pageview_start_tstamp)
-> Parallel Seq Scan on celebrus_fact_agg_1_p2015_12
(cost=0.00..743769.76 rows=4221741 width=12) (actual time=0.066..1631
.650 rows=3618887 loops=7)

One question - how is the upper limit of workers chosen?

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 Mon, Mar 14, 2016 at 12:30 PM, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com
> wrote:

> On 14 March 2016 at 14:16, James Sewell <james(dot)sewell(at)lisasoft(dot)com> wrote:
>
>> 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)
>>
>
> Great! Thanks for testing this.
>
> If you run EXPLAIN ANALYZE on this with the 6 workers, does the actual
> number of Gather rows come out at 105? I'd just like to get an idea of my
> cost estimate for the Gather are going to be accurate for real world data
> sets.
>
>
> --
> David Rowley http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services
>

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