MySQL的伪行级锁


接触mysql之前一直以为mysql的innodb引擎所支持的行级锁和Oracle，postgresql是一样的，是对数据行上加锁。但其实是不一样的，理解不一样，对mysql的锁机制就容易产生误解。innodb的行级锁实际上是基于索引项来锁定的。以下是测试机上的验证测试过程 

一.数据准备 

mysql> use test;
Database changed
mysql> show create table t_kenyon \G
*************************** 1. row ***************************
       Table: t_kenyon
Create Table: CREATE TABLE `t_kenyon` (
  `id` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> set autocommit = 0;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like '%autocommit%';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set (0.00 sec)

mysql> show variables like '%innodb_lock%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| innodb_lock_wait_timeout       | 50    | 
| innodb_locks_unsafe_for_binlog | OFF   | 
+--------------------------------+-------+
2 rows in set (0.00 sec)

mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)

mysql> select * from t_kenyon;
+------+
| id   |
+------+
|    1 |
|  123 |
|  789 |
|  345 |
|   78 |
|   78 |
+------+
6 rows in set (0.00 sec)
以上是测试表t_kenyon,设置提交方式为手动提交.

二.过程(开启两个session,分别设置autocommit=off)

1.session 1 update 
mysql> update t_kenyon set id = 999 where id = 1;
Query OK, 1 row affected (0.04 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t_kenyon;
+------+
| id   |
+------+
|  999 |
|  123 |
|  789 |
|  345 |
|   78 |
|   78 |
+------+
6 rows in set (0.00 sec)
2.session 2 update 
mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set (0.00 sec)

mysql> select * from t_kenyon;
+------+
| id   |
+------+
|    1 |
|  123 |
|  789 |
|  345 |
|   78 |
|   78 |
+------+
6 rows in set (0.00 sec)

mysql> update t_kenyon set id = 88888 where id = 345;
第二个session更新的值是345，但是也一直被阻塞，直到session1被rollback或者commit,如果session1未做回滚或者提交，session2中的该阻塞在超出mysql的锁时间限制时自动回滚,该参数为innodb_lock_wait_timeout，默认值50秒 现象如下 
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

加索引后的测试 
3.session 1 update

mysql> create index ind_kenyon on t_kenyon(id);
Query OK, 0 rows affected (28.58 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> update t_kenyon set id = 999 where id = 1;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t_kenyon;
+------+
| id   |
+------+
|   78 |
|   78 |
|  123 |
|  345 |
|  789 |
|  999 |
+------+
6 rows in set (0.00 sec)
4.session 2 update 
 mysql> select * from t_kenyon;
+------+
| id   |
+------+
|    1 |
|   78 |
|   78 |
|  123 |
|  345 |
|  789 |
+------+
6 rows in set (0.00 sec)

mysql> update t_kenyon set id = 7777 where id = 345;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from t_kenyon;
+------+
| id   |
+------+
|    1 |
|   78 |
|   78 |
|  123 |
|  789 |
| 7777 |
+------+
6 rows in set (0.00 sec)
执行计划 
mysql> explain select * from t_kenyon where id = 345 \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t_kenyon
         type: ref
possible_keys: ind_kenyon
          key: ind_kenyon
      key_len: 5
          ref: const
         rows: 1
        Extra: Using where; Using index
1 row in set (0.00 sec)
可以看到加了索引后，不同的数据更新并没有被阻塞，实现了真正意义上行锁
三.行级锁的扩展限制
1.相同索引阻塞 
mysql> select * from t_kenyon;
+------+---------+
| id   | name    |
+------+---------+
|    1 | kenyon  |
|  123 | francs  |
|  789 | lighten |
|  345 | mood    |
|   78 | opp     |
|   78 | opp     |
|  789 | james   |
+------+---------+
7 rows in set (0.00 sec)

mysql> explain select * from t_kenyon where id =1 \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t_kenyon
         type: ref
possible_keys: ind_kenyon
          key: ind_kenyon
      key_len: 5
          ref: const
         rows: 1
        Extra: Using where
1 row in set (0.00 sec)

mysql> update t_kenyon set name = 'john' where id = 789 and name = 'james';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0
2.在另外的会话中同样对id=789的另一条数据进行更新 
mysql> update t_kenyon set name = 'yagobu' where id = 789 and name ='lighten';
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

mysql> update t_kenyon set name = 'yagobu' where id = 789 and name ='xxxxx';
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
这时我们可以看到即使有索引，不同数据的更新也被阻塞了,哪怕没有检索到其他数据，只要用到了同一个索引键值都会被阻塞,如上述name='xxxx'中是没有的，也被阻塞了。 所以可以引申开来，只要是在一个session中>或者<某个值的查询中索引项被扫到了，都将产生锁，并阻塞其他事务对之前锁中内容的任何更新。

3.不同事务对表的不同索引锁定不同行
a.session 1 
mysql> select * from t_kenyon;
+------+------------+
| id   | name       |
+------+------------+
|    1 | kenyon     |
|  123 | francs     |
|  345 | mood       |
|   78 | opp        |
|   78 | opp        |
|  789 | lighten    |
|  789 | james      |
|  899 | jiangkaish |
|  902 | song       |
|  907 | hu         |
|  997 | wenjiab    |
+------+------------+
11 rows in set (0.00 sec)

mysql> update t_kenyon set name = 'kenyon_god' where id = 789;
Query OK, 2 rows affected (0.01 sec)
Rows matched: 2  Changed: 2  Warnings: 0
2.session 2更新另一个 
mysql> update t_kenyon set name = 'koko' where name = 'hu';
3.在session 1中查看进程 
mysql> show processlist;
+----+------+-----------+------+---------+------+----------+-----------------------------------------------------+
| Id | User | Host      | db   | Command | Time | State    | Info                                                |
+----+------+-----------+------+---------+------+----------+-----------------------------------------------------+
|  8 | root | localhost | test | Query   |    0 | NULL     | show processlist                                    |
|  9 | root | localhost | test | Query   |    8 | Updating | update t_kenyon set name = 'koko' where name = 'hu' |
| 10 | root | localhost | NULL | Sleep   |  113 |          | NULL                                                |
+----+------+-----------+------+---------+------+----------+-----------------------------------------------------+
3 rows in set (0.00 sec)
可以看到被锁住了。

4.对name项加索引达到目的
a.会话1 
mysql> alter table t_kenyon add index ind(name);
Query OK, 0 rows affected (0.33 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> update t_kenyon set name = 'kenyon_god' where id = 789;
Query OK, 2 rows affected (0.02 sec)
Rows matched: 2  Changed: 2  Warnings: 0
b.会话2 
mysql> update t_kenyon set name = 'koko' where name = 'hu';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

至此对表的不同行数据也达到了更新的目的。

四.总结
MYSQL的innodb引擎在查询过程中如果有基于索引扫描，可以实现行级锁定，但是该行级锁有相当的限制，查询未使用索引或表中未建索引时会触发表锁，数据量大、查询效果慢的情况下该现象将被放大，严重会导致DB奔溃。

相关内容