謹慎使用MYSQL表分區

test1表是未分區

test2表是hash分區

如下是兩個表的表結構：

mysql> show create table test1;
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                      |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test1 | CREATE TABLE `test1` (
  `id` int(11) NOT NULL,
  `tid` int(11) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `tid` (`tid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

 

mysql> show create table test2;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                       |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| test2 | CREATE TABLE `test2` (
  `id` int(11) NOT NULL,
  `tid` int(11) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `tid` (`tid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
/*!50100 PARTITION BY HASH (id)
PARTITIONS 10 */ |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)

在主鍵id列上用上hash分區。

 

若是是在主鍵上查詢，那麼分區要比不分區性能高，例如：

mysql> explain partitions select * from test1 where id in (86,777,432345);
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | test1 | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.06 sec)

mysql> explain partitions select * from test2 where id in (86,777,432345);
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | test2 | p5,p6,p7   | range | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.01 sec)
 

只在分區P5/6/7上就掃描到了記錄，減小了過多的磁盤IO。

但若是在索引tid上查詢，性能就很低了。

mysql>  explain partitions select * from test1 where tid = 86419804;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | test1 | NULL       | ref  | tid           | tid  | 5       | const |    1 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

mysql>  explain partitions select * from test2 where tid = 86419804;
+----+-------------+-------+-------------------------------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | partitions                    | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+-------+-------------------------------+------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | test2 | p0,p1,p2,p3,p4,p5,p6,p7,p8,p9 | ref  | tid           | tid  | 5       | const |   10 | Using where |
+----+-------------+-------+-------------------------------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)
 

要掃描10個分區，才能獲得結果。

mysql> select sql_no_cache * from test1 where tid=86419804;
+----+----------+------+
| id | tid      | name |
+----+----------+------+
| 11 | 86419804 | abc2 |
+----+----------+------+
1 row in set (0.40 sec)

mysql> select sql_no_cache * from test2 where tid=86419804;
+----+----------+------+
| id | tid      | name |
+----+----------+------+
| 11 | 86419804 | abc2 |
+----+----------+------+
1 row in set (1.98 sec)

分區後的查詢結果要比未分區的慢不少。

結論：分區規則僅限制在主鍵上使用，性能會有提升，在其餘字段上使用，即便有索引，性能也會不好。