hash agg is slower on wide tables?

Hi

I did some benchmarks and I found some strange numbers.

do $$
begin
drop table if exists t1;
execute 'create table t1(' ||
array_to_string(array(select 'a' || i || ' smallint' from
generate_series(1,30) g(i)), ',') || ')';
-- special column a2, a11
insert into t1
select 2008, i % 12 + 1, random()*20, random()*20, random()*20,
random()*20, random()*20, random()*20, random()*20, random()*20,
case when random() < 0.9 then 1 else 0 end, random()*20,
random()*20, random()*20, random()*20, random()*20, random()*20,
random()*20, random()*20, random()*20,
random()*20, random()*20, random()*20, random()*20, random()*20,
random()*20, random()*20, random()*20, random()*20, random()*20
from generate_series(1,7009728) g(i);
drop table if exists t2;
create table t2 as select a2, a11 from t1;
analyze t1; analyze t2;
end;
$$;

postgres=# \dt+ t*
List of relations
Schema | Name | Type | Owner | Size | Description
--------+------+-------+-------+--------+-------------
public | t1 | table | pavel | 622 MB |
public | t2 | table | pavel | 242 MB |
(2 rows)

postgres=# explain analyze select count(*), a2, a11 from t1 group by 3,2
order by 3,2;
QUERY
PLAN
---------------------------------------------------------------------------------------------------------------------------
Sort (cost=202327.03..202327.09 rows=24 width=4) (actual
time=2609.159..2609.161 rows=24 loops=1)
Sort Key: a11, a2
Sort Method: quicksort Memory: 26kB
-> HashAggregate (cost=202326.24..202326.48 rows=24 width=4) (actual
time=2609.137..2609.139 rows=24 loops=1) --- grouping 1997 ms
Group Key: a11, a2
-> Seq Scan on t1 (cost=0.00..149753.28 rows=7009728 width=4)
(actual time=0.071..616.222 rows=7009728 loops=1)
Planning time: 0.138 ms
Execution time: 2609.247 ms
(8 rows)

postgres=# explain analyze select count(*), a2, a11 from t2 group by 3,2
order by 3,2;
QUERY
PLAN
---------------------------------------------------------------------------------------------------------------------------
Sort (cost=153688.03..153688.09 rows=24 width=4) (actual
time=2100.058..2100.059 rows=24 loops=1)
Sort Key: a11, a2
Sort Method: quicksort Memory: 26kB
-> HashAggregate (cost=153687.24..153687.48 rows=24 width=4) (actual
time=2100.037..2100.040 rows=24 loops=1) --- grouping 1567 ms -- 25% faster
Group Key: a11, a2
-> Seq Scan on t2 (cost=0.00..101114.28 rows=7009728 width=4)
(actual time=0.043..532.680 rows=7009728 loops=1)
Planning time: 0.178 ms
Execution time: 2100.158 ms
(8 rows)

postgres=# \dt+ t*
List of relations
Schema | Name | Type | Owner | Size | Description
--------+------+-------+-------+---------+-------------
public | t1 | table | pavel | 6225 MB |
public | t2 | table | pavel | 2423 MB |
(2 rows)

postgres=# explain analyze select count(*), a2, a11 from t1 group by 3,2
order by 3,2;
QUERY
PLAN
---------------------------------------------------------------------------------------------------------------------------------
Sort (cost=2023263.19..2023263.25 rows=24 width=4) (actual
time=99453.272..99453.274 rows=24 loops=1)
Sort Key: a11, a2
Sort Method: quicksort Memory: 26kB
-> HashAggregate (cost=2023262.40..2023262.64 rows=24 width=4) (actual
time=99453.244..99453.249 rows=24 loops=1) --- 31891 ms
Group Key: a11, a2
-> Seq Scan on t1 (cost=0.00..1497532.80 rows=70097280 width=4)
(actual time=16.935..67562.615 rows=70097280 loops=1)
Planning time: 14.526 ms
Execution time: 99453.413 ms
(8 rows)

postgres=# explain analyze select count(*), a2, a11 from t2 group by 3,2
order by 3,2;
QUERY
PLAN
-------------------------------------------------------------------------------------------------------------------------------
Sort (cost=1536868.33..1536868.39 rows=24 width=4) (actual
time=20656.397..20656.399 rows=24 loops=1)
Sort Key: a11, a2
Sort Method: quicksort Memory: 26kB
-> HashAggregate (cost=1536867.54..1536867.78 rows=24 width=4) (actual
time=20656.375..20656.378 rows=24 loops=1) --- 15248 ms --100% faster
Group Key: a11, a2
-> Seq Scan on t2 (cost=0.00..1011137.88 rows=70097288 width=4)
(actual time=0.060..5408.205 rows=70097280 loops=1)
Planning time: 0.161 ms
Execution time: 20656.475 ms
(8 rows)

It looks like hah agg is slower when it is based on wide table about
25-100%. Is it - or I don't see something?

Regards

Pavel