Shared and Exclusive Locks 共享和排它锁

14.5 InnoDB Locking and Transaction Model InnoDB 锁和事务模型

14.5.1 InnoDB Locking
14.5.2 InnoDB Transaction Model
14.5.3 Locks Set by Different SQL Statements in InnoDB
14.5.4 Phantom Rows
14.5.5 Deadlocks in InnoDB

实现一个大规模的,繁忙的,或者可扩展的数据库应用,从不同数据库系统的大量代码,或者调整MySQL 性能,

它是重要的了解InnoDB 锁和InnoDB  事务模型

本节讨论几个议题关于InnoDB 锁和InnoDB 事务模型:

14.5.1 InnoDB Locking

这个章节描述InnoDB使用的锁的类型:

    Shared and Exclusive Locks

    Intention Locks

    Record Locks

    Gap Locks

    Next-Key Locks

    Insert Intention Locks

    AUTO-INC Locks 

Shared and Exclusive Locks 共享和排它锁

InnoDB 实现标准的航机所 有两种类型的锁,shared (S) locks and exclusive (X) locks.  共享和排它锁

1.一个共享锁允许事务是有一个锁来读取一行

2.一个排它锁允许事务持有锁来更新或者删除记录

如果事务T1 持有一个共享锁在记录r, 另外一个事务T2 请求一个lock 在记录r 是如下处理:

1.T2请求S锁可以立即被授予,作为结果,T1和T2 都持有S锁在记录r上

2. T2请求X锁不能马上授权

如果一个事务是有一个排它锁在记录r上,从其他事务T2请求任何类型的锁都不能被立即授予。

相反, 事务T2 只能等待事务T1释放它在记录r上的锁

·共享锁（S）：SELECT * FROM table_name WHERE ... LOCK IN SHARE MODE。

·排他锁（X)：SELECT * FROM table_name WHERE ... FOR UPDATE。

模式:

第一种情况 session1 持有共享锁:

Session 1加共享锁:

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from aa where username='admin' LOCK IN SHARE MODE;
+----------+----------------------------------+-----------+
| username | password                         | authority |
+----------+----------------------------------+-----------+
| admin    | 0192023a7bbd73250516f069df18b500 |         1 |
+----------+----------------------------------+-----------+
1 row in set (0.00 sec)

Session 2此时也可以加共享锁:

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql>  select * from aa where username='admin' LOCK IN SHARE MODE;
+----------+----------------------------------+-----------+
| username | password                         | authority |
+----------+----------------------------------+-----------+
| admin    | 0192023a7bbd73250516f069df18b500 |         1 |
+----------+----------------------------------+-----------+
1 row in set (0.00 sec)

mysql>  select * from aa where username='admin' for update;

加排它锁会HANG

第一种情况 session2 持有排它锁:

Session 1持有排它锁:

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from aa where username='admin' for update;
+----------+----------------------------------+-----------+
| username | password                         | authority |
+----------+----------------------------------+-----------+
| admin    | 0192023a7bbd73250516f069df18b500 |         1 |
+----------+----------------------------------+-----------+
1 row in set (0.00 sec)

Session 2:

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from aa where username='admin' LOCK IN SHARE MODE;

无法获取共享锁

也无法获取排它锁