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
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: 02.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: 02.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: 0b.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。
