executor/aggregate: convert int to decimal in sum

[AndreMouche]

@XuHuaiyu @BusyJay PTAL

[ngaut]

Why make the change?

[AndreMouche]

This PR is used to fix this issue pingcap/tidb#5643

[AndreMouche]

Performace test

1 tidb/3 tikv/1pd on one machine

table schema

desc testagg;
+-----------------+---------------+------+------+---------+-------+
| Field           | Type          | Null | Key  | Default | Extra |
+-----------------+---------------+------+------+---------+-------+
| a      | int(11)       | NO   | PRI  | NULL    |       |
| b       | int(11)       | NO   |      | NULL    |       |
| c       | int(11)       | NO   |      | NULL    |       |
| d       | int(11)       | NO   | PRI  | NULL    |       |

records count

select count(*) from testagg;
+----------+
| count(*) |
+----------+
|  6001215 |
+----------+
1 row in set (1.69 sec

test with master

mysql> select sum(a),sum(b),sum(d),sum(c) from testagg;
+-----------------+----------------+-------------------+----------------+
| sum(a) | sum(b) | sum(d) | sum(c) |
+-----------------+----------------+-------------------+----------------+
|  18005322964949 |   600229457837 |          18007100 |    30009691369 |
+-----------------+----------------+-------------------+----------------+
1 row in set (3.70 sec)

mysql> select sum(a),sum(b),sum(d),sum(c) from testagg;
+-----------------+----------------+-------------------+----------------+
| sum(a) | sum(b) | sum(d) | sum(c) |
+-----------------+----------------+-------------------+----------------+
|  18005322964949 |   600229457837 |          18007100 |    30009691369 |
+-----------------+----------------+-------------------+----------------+
1 row in set (3.62 sec)

mysql> select sum(a),sum(b),sum(d),sum(c) from testagg;
+-----------------+----------------+-------------------+----------------+
| sum(a) | sum(b) | sum(d) | sum(c) |
+-----------------+----------------+-------------------+----------------+
|  18005322964949 |   600229457837 |          18007100 |    30009691369 |
+-----------------+----------------+-------------------+----------------+
1 row in set (3.55 sec)

test with current branch

mysql> select sum(a),sum(b),sum(d),sum(c) from testagg;
+-----------------+----------------+-------------------+----------------+
| sum(a) | sum(b) | sum(d) | sum(c) |
+-----------------+----------------+-------------------+----------------+
|  18005322964949 |   600229457837 |          18007100 |    30009691369 |
+-----------------+----------------+-------------------+----------------+
1 row in set (3.70 sec)

mysql> select sum(a),sum(b),sum(d),sum(c) from testagg;
+-----------------+----------------+-------------------+----------------+
| sum(a) | sum(b) | sum(d) | sum(c) |
+-----------------+----------------+-------------------+----------------+
|  18005322964949 |   600229457837 |          18007100 |    30009691369 |
+-----------------+----------------+-------------------+----------------+
1 row in set (3.82 sec)

mysql> select sum(a),sum(b),sum(d),sum(c) from testagg;
+-----------------+----------------+-------------------+----------------+
| sum(a) | sum(b) | sum(d) | sum(c) |
+-----------------+----------------+-------------------+----------------+
|  18005322964949 |   600229457837 |          18007100 |    30009691369 |
+-----------------+----------------+-------------------+----------------+
1 row in set (3.80 sec)

[siddontang]

performance reduces?

[AndreMouche]

Yes, since the decimal's computation is slower than the integer @siddontang

[XuHuaiyu]

LGTM

[AndreMouche]

@BusyJay PTAL

[AndreMouche]

@BusyJay @hicqu PTAL

[BusyJay]

LGTM