Re: [HACKERS] Partition-wise aggregation/grouping

On Fri, Oct 13, 2017 at 12:06 PM, Jeevan Chalke
<jeevan.chalke@enterprisedb.com> wrote:
>
While playing around with the patch I have noticed one regression with
the partial partition-wise aggregate.

I am consistently able to reproduce this on my local machine.

Scenario: Group by on non-key column and only one tuple per group.

Complete Test:
--------------------
create table t(a int,b int) partition by range(a);
create table t1 partition of t for values from (1) to (100000);
create table t2 partition of t for values from (100000) to (200000);

insert into t values (generate_series(1,199999),generate_series(1, 199999));
postgres=# explain analyze select sum(a) from t group by b;
QUERY
PLAN

--------------------------------------------------------------------------------------------------------------------------------------Finalize
GroupAggregate (cost=20379.55..28379.51 rows=199999
 
width=12) (actual time=102.311..322.969 rows=199999 loops=1)  Group Key: t1.b  ->  Merge Append
(cost=20379.55..25379.53rows=199999 width=12)
 
(actual time=102.303..232.310 rows=199999 loops=1)        Sort Key: t1.b        ->  Partial GroupAggregate
(cost=10189.72..11939.70
rows=99999 width=12) (actual time=52.164..108.967 rows=99999 loops=1)              Group Key: t1.b              ->
Sort (cost=10189.72..10439.72 rows=99999 width=8)
 
(actual time=52.158..66.236 rows=99999 loops=1)                    Sort Key: t1.b                    Sort Method:
externalmerge  Disk: 1768kB                    ->  Seq Scan on t1  (cost=0.00..1884.99
 
rows=99999 width=8) (actual time=0.860..20.388 rows=99999 loops=1)        ->  Partial GroupAggregate
(cost=10189.82..11939.82
rows=100000 width=12) (actual time=50.134..102.976 rows=100000
loops=1)              Group Key: t2.b              ->  Sort  (cost=10189.82..10439.82 rows=100000 width=8)
(actual time=50.128..63.362 rows=100000 loops=1)                    Sort Key: t2.b                    Sort Method:
externalmerge  Disk: 1768kB                    ->  Seq Scan on t2  (cost=0.00..1885.00
 
rows=100000 width=8) (actual time=0.498..20.977 rows=100000 loops=1)Planning time: 0.190 msExecution time: 339.929 ms
(18 rows)

postgres=# set enable_partition_wise_agg=off;
SET
postgres=# explain analyze select sum(a) from t group by b;
QUERYPLAN
 

----------------------------------------------------------------------------------------------------------------------------------GroupAggregate
(cost=26116.53..29616.51 rows=199999 width=12)
 
(actual time=139.413..250.751 rows=199999 loops=1)  Group Key: t1.b  ->  Sort  (cost=26116.53..26616.52 rows=199999
width=8)(actual
 
time=139.406..168.775 rows=199999 loops=1)        Sort Key: t1.b        Sort Method: external merge  Disk: 3544kB
->  Result  (cost=0.00..5769.98 rows=199999 width=8) (actual
 
time=0.674..76.392 rows=199999 loops=1)              ->  Append  (cost=0.00..3769.99 rows=199999 width=8)
(actual time=0.672..40.291 rows=199999 loops=1)                    ->  Seq Scan on t1  (cost=0.00..1884.99
rows=99999 width=8) (actual time=0.672..12.408 rows=99999 loops=1)                    ->  Seq Scan on t2
(cost=0.00..1885.00
rows=100000 width=8) (actual time=1.407..11.689 rows=100000 loops=1)Planning time: 0.146 msExecution time: 263.678 ms
(11 rows)


-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com


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