linux Mysql 事务隔离级别测试
Mysql 5.5默认使用innodb存储引擎（表类型），innodb是支持事务处理的，对innodb下各事务隔离级别进行测试，可以更好的理解各级别产生的各种问题~
设置innodb的事务级别方法是：set 作用域 transaction isolation level 事务隔离级别，例如~
mysql > set global transaction isolation level read committed; //全局的
mysql > set session transaction isolation level read committed; //当前会话
测试使用的表名为student ,表结构为id(int)，name(varchar(16))，age(int)~
1.测试未授权读取（read uncommitted）：允许脏读取，但不允许更新丢失
事务T1：
mysql> select @@tx_isolation;
+------------------+
| @@tx_isolation |
+------------------+
| READ-UNCOMMITTED |
+------------------+
1 row in set (0.00 sec)
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
+----+------+-----+
2 rows in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> update student set name = 'tom' where id = 2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
事务T2：
mysql> select @@tx_isolation;
+------------------+
| @@tx_isolation |
+------------------+
| READ-UNCOMMITTED |
+------------------+
1 row in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
事务T1：
mysql> update student set name = 'tom' where id = 2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
事务T2：
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | tom | 18 |
+----+------+-----+
2 rows in set (0.00 sec)
事务T1：
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)
事务T2：
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
+----+------+-----+
2 rows in set (0.00 sec)
以上可以看出，脏读了~
事务T2：
mysql> update student set name = 'bob' where id = 2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
事务T1：
mysql> update student set name = 'tom' where id = 2;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
以上可以看出事务T1和事务T2之间更新没有丢失，直到一个更新提交后另一个更新才能进行，这里发生了等待超时错误~
2.测试授权读取（read committed）：允许不可重复读取，但不允许脏读取
事务T1：
mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set (0.00 sec)
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
+----+------+-----+
2 rows in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
事务T2：
mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set (0.00 sec)
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
+----+------+-----+
2 rows in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
事务T1：
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
+----+------+-----+
2 rows in set (0.00 sec)
事务T2：
mysql> insert into student values(3,'bob',20);
Query OK, 1 row affected (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
事务T1:
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | bob | 20 |
+----+------+-----+
3 rows in set (0.00 sec)
可以看出，在T1事务中，条件相同的两次读取出现不同的结果~~脏读测试这里没有进行，可以自己测试看看~
3.可重复读取(Repeatable Read)：禁止不可重复读取和脏读取，但是有时可能出现幻影数据，innodb中使用了措施来防止幻读
事务T1：
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | bob | 20 |
+----+------+-----+
3 rows in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
事务T2：
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | bob | 20 |
+----+------+-----+
3 rows in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
事务T2：
mysql> update student set name = 'mary' where id = 3;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
事务T1：
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | bob | 20 |
+----+------+-----+
3 rows in set (0.00 sec)
可以看出，没有出现脏读~
事务T2：
mysql> insert into student values(4,'lucy',18);
Query OK, 1 row affected (0.00 sec
事务T1：
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | bob | 20 |
+----+------+-----+
3 rows in set (0.00 sec)
事务T2：
mysql> commit;
Query OK, 0 rows affected (0.01 sec)
事务T1：
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | bob | 20 |
+----+------+-----+
3 rows in set (0.00 sec)
可以看出，在同一个事务中没有出现不可重复读（更新提交后）和幻读（插入提交后）~
4.序列化(Serializable)：提供严格的事务隔离，锁表，读取时会自动设置读锁，然后所有会话只能进行读操作；写时自动设置写锁，然后所有会话只能等待~
事务T1：
mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| SERIALIZABLE |
+----------------+
1 row in set (0.00 sec)
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | mary | 20 |
| 4 | lily | 18 |
+----+------+-----+
4 rows in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
事务T2：
mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| SERIALIZABLE |
+----------------+
1 row in set (0.00 sec)
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | mary | 20 |
| 4 | lily | 18 |
+----+------+-----+
4 rows in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
事务T2：
mysql> update student set name = 'lucy' where id = 4;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
事务T1：
mysql> select * from student;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
事务T1：
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from student;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
从上可以看出,T2存在写锁，所有T2的读操作会等待超时~
事务T2：
mysql> COMMIT;
Query OK, 0 rows affected (0.01 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
事务T1：
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | mary | 20 |
| 4 | lucy | 18 |
+----+------+-----+
4 rows in set (0.01 sec)
事务T2：
mysql> select * from student;
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | kate | 18 |
| 2 | jim | 18 |
| 3 | mary | 20 |
| 4 | lucy | 18 |
+----+------+-----+
4 rows in set (0.00 sec)
从上面可以看出，T1设置了读锁，T2还可以继续读取
事务T2：
mysql> update student set name = 'bob' where id = 2;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
从上面可以看出，T1设置了读锁，T2不能进行写操作，写操作等待超时~