Re: Partition-wise aggregation/grouping

On Fri, Oct 13, 2017 at 12:06 PM, Jeevan Chalke
<jeevan(dot)chalke(at)enterprisedb(dot)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..25379.53 rows=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: external merge 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: external merge 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 ms
Execution 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;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------
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 ms
Execution time: 263.678 ms
(11 rows)

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