InnoDB有三种行锁的算法:
1,Record Lock:单个行记录上的锁。
2,Gap Lock:间隙锁,锁定一个范围,但不包括记录本身。
3,Next-Key Lock:1+2,锁定一个范围,并且锁定记录本身。对于行的查询,都是采用该方法,主要目的是解决幻读的问题。
测试一:
root@localhost : test 10:56:10>create table t(a int,key idx_a(a))engine =innodb;
Query OK, 0 rows affected (0.20 sec)
root@localhost : test 10:56:13>insert into t values(1),(3),(5),(8),(11);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
root@localhost : test 10:56:15>select * from t;
+------+
| a |
+------+
| 1 |
| 3 |
| 5 |
| 8 |
| 11 |
+------+
5 rows in set (0.00 sec)
section A:
root@localhost : test 10:56:27>start transaction;
Query OK, 0 rows affected (0.00 sec)
root@localhost : test 10:56:29>select * from t where a = 8 for update;
+------+
| a |
+------+
| 8 |
+------+
1 row in set (0.00 sec)
section B:
root@localhost : test 10:54:50>begin;
Query OK, 0 rows affected (0.00 sec)
root@localhost : test 10:56:51>select * from t;
+------+
| a |
+------+
| 1 |
| 3 |
| 5 |
| 8 |
| 11 |
+------+
5 rows in set (0.00 sec)
root@localhost : test 10:56:54>insert into t values(2);
Query OK, 1 row affected (0.00 sec)
root@localhost : test 10:57:01>insert into t values(4);
Query OK, 1 row affected (0.00 sec)
++++++++++
root@localhost : test 10:57:04>insert into t values(6);
root@localhost : test 10:57:11>insert into t values(7);
root@localhost : test 10:57:15>insert into t values(9);
root@localhost : test 10:57:33>insert into t values(10);
++++++++++
上面全被锁住,阻塞住了
root@localhost : test 10:57:39>insert into t values(12);
Query OK, 1 row affected (0.00 sec)
问题:
为什么section B上面的插入语句会出现锁等待的情况?InnoDB是行锁,在section A里面锁住了a=8的行,其他应该不受影响。why?
分析:
因为InnoDB对于行的查询都是采用了Next-Key Lock的算法,锁定的不是单个值,而是一个范围。上面索引有1,3,5,8,11,被Next-Key Locking的区间为:
(-∞,1),(1,3],(3,5],(5,8],(8,11],(11,+∞)
特别需要注意的是,InnoDB存储引擎还会对辅助索引下一个键值加上gap lock。如上面分析,那就可以解释了。
root@localhost : test 10:56:29>select * from t where a = 8 for update;
+------+
| a |
+------+
| 8 |
+------+
1 row in set (0.00 sec)
该SQL语句锁定的范围是(5,8],下个下个键值范围是(8,11],所以插入5~11之间的值的时候都会被锁定,要求等待。即:插入5,6,7,8,9,10 会被锁住。插入非这个范围内的值都正常。
继续:插入超时失败后,会怎么样?
超时时间的参数:innodb_lock_wait_timeout ,默认是50秒。
超时是否回滚参数:innodb_rollback_on_timeout 默认是OFF。
section A:
root@localhost : test 04:48:51>start transaction;
Query OK, 0 rows affected (0.00 sec)
root@localhost : test 04:48:53>select * from t where a = 8 for update;
+------+
| a |
+------+
| 8 |
+------+
1 row in set (0.01 sec)
section B:
root@localhost : test 04:49:04>start transaction;
Query OK, 0 rows affected (0.00 sec)
root@localhost : test 04:49:07>insert into t values(12);
Query OK, 1 row affected (0.00 sec)
root@localhost : test 04:49:13>insert into t values(10);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
root@localhost : test 04:50:06>select * from t;
+------+
| a |
+------+
| 1 |
| 3 |
| 5 |
| 8 |
| 11 |
| 12 |
+------+
6 rows in set (0.00 sec)
经过测试,不会回滚超时引发的异常,当参数innodb_rollback_on_timeout 设置成ON时,则可以回滚,会把插进去的12回滚掉。
默认情况下,InnoDB存储引擎不会回滚超时引发的异常,除死锁外。
既然InnoDB有三种算法,那Record Lock什么时候用?还是用上面的列子,把辅助索引改成唯一属性的索引。
测试二:
root@localhost : test 04:58:49>create table t(a int primary key)engine =innodb;
Query OK, 0 rows affected (0.19 sec)
root@localhost : test 04:59:02>insert into t values(1),(3),(5),(8),(11);
Query OK, 5 rows affected (0.00 sec)
Records: 5 Duplicates: 0 Warnings: 0
root@localhost : test 04:59:10>select * from t;
+----+
| a |
+----+
| 1 |
| 3 |
| 5 |
| 8 |
| 11 |
+----+
5 rows in set (0.00 sec)
section A:
root@localhost : test 04:59:30>start transaction;
Query OK, 0 rows affected (0.00 sec)
root@localhost : test 04:59:33>select * from t where a = 8 for update;
+---+
| a |
+---+
| 8 |
+---+
1 row in set (0.00 sec)
section B:
root@localhost : test 04:58:41>start transaction;
Query OK, 0 rows affected (0.00 sec)
root@localhost : test 04:59:45>insert into t values(6);
Query OK, 1 row affected (0.00 sec)
root@localhost : test 05:00:05>insert into t values(7);
Query OK, 1 row affected (0.00 sec)
root@localhost : test 05:00:08>insert into t values(9);
Query OK, 1 row affected (0.00 sec)
root@localhost : test 05:00:10>insert into t values(10);
Query OK, 1 row affected (0.00 sec)
问题:
为什么section B上面的插入语句可以正常,和测试一不一样?
分析:
因为InnoDB对于行的查询都是采用了Next-Key Lock的算法,锁定的不是单个值,而是一个范围,按照这个方法是会和第一次测试结果一样。但是,当查询的索引含有唯一属性的时候,Next-Key Lock 会进行优化,将其降级为Record Lock,即仅锁住索引本身,不是范围。
如何让测试一不阻塞?可以显式的关闭Gap Lock:
1:把事务隔离级别改成:Read Committed,提交读、不可重复读。SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
2:修改参数:innodb_locks_unsafe_for_binlog 设置为1。