深入理解mysql的隔离级别

建表插入测试数据
A> create table test(id int ,num int) ;
Query OK, 0 rows affected (0.53 sec)

A> insert into test values(1,1);
Query OK, 1 row affected (0.01 sec)

A> insert into test values(2,2);
Query OK, 1 row affected (0.00 sec)

A> insert into test values(3,3);
Query OK, 1 row affected (0.01 sec)

A> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

A> show create table test ;
+-------+----------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+----------------------------------------------------------------------------------------------------------------------+
| test | CREATE TABLE `test` (
`id` int(11) DEFAULT NULL,
`num` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+----------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

一。隔离级别read uncommitted，事物A可以读到事物B的未提交数据，未提交数据称为脏数据，因此叫脏读。
A> set session transaction isolation level read uncommitted ;
Query OK, 0 rows affected (0.00 sec)

A> SELECT @@tx_isolation ;
+------------------+
| @@tx_isolation |
+------------------+
| READ-UNCOMMITTED |
+------------------+
1 row in set (0.00 sec)

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

A> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

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

B> update test set num=10 where id=1 ;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0

B> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 10 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

A> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 10 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.01 sec)

B> rollback ;
Query OK, 0 rows affected (0.01 sec)

B> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

A> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

二。隔离级别read committed ，
对于事物B未提交的DML操作，事物A是看不到的
但是事物A在本身事物操作的过程中（未做commit or rollback)，可以读到事物B的已提交数据，这就是不可重复读
下面测试下：
A> set session transaction isolation level read committed ;
Query OK, 0 rows affected (0.00 sec)

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

A> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

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

B>update test set num=20 where id=1 ;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

B>rollback ;
Query OK, 0 rows affected (0.01 sec)

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

B>insert into test values(4,4) ;
Query OK, 1 row affected (0.00 sec)

B>select * from test;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
+------+------+
4 rows in set (0.00 sec)

B>rollback ;
Query OK, 0 rows affected (0.01 sec)

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

B>delete from test where id =1 ;
Query OK, 1 row affected (0.01 sec)

B>select * from test;
+------+------+
| id | num |
+------+------+
| 2 | 2 |
| 3 | 3 |
+------+------+
2 rows in set (0.00 sec)

B>rollback ;
Query OK, 0 rows affected (0.01 sec)

在事物B做这些DML操作的时候，事物A的查询结果一直没变，此时事物A尚未结束
A> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

提交事物B的某个操作，事物A里面的对表test的查询结果就变了，因此事物A里面对test表的查询是不可重复的，所以叫不可重复读

三。隔离级别repeatable read
mysql的默认事物隔离级别，防止了脏读和不可重复读，但可能出现幻读
该隔离级别解决了不重复读，保证了同一个事务里，查询的结果都是事务开始时的状态（一致性）。
但是，如果另一个事务同时提交了新数据，本事务再更新时，就会“惊奇的”发现了这些新数据，貌似之前读到的数据是“鬼影”一样的幻觉。

A> set session transaction isolation level repeatable read ;
Query OK, 0 rows affected (0.00 sec)

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

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

A> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

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

B> update test set num=10 where id=1 ;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

B> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 10 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

B> commit ;
Query OK, 0 rows affected (0.01 sec)

此处A的修改应该是num=3才对，但是因为幻读，事物B虽然提交了，但事物A读到的还是旧记录，
如同幻影一般，事物A修改后，发现结果是30，而不是想要的结果3
A> update test set num=num*3 where id=1 ;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1 Changed: 1 Warnings: 0

A> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 30 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

A> commit ;
Query OK, 0 rows affected (0.01 sec)

四。隔离级别SERIALIZABLE
SERIALIZABLE事务隔离级别最严厉，在进行查询时就会对表或行加上共享锁，其他事务对该表将只能进行读操作，而不能进行写操作。

A> set session transaction isolation level serializable ;
Query OK, 0 rows affected (0.00 sec)

A> select @@session.tx_isolation ;
+------------------------+
| @@session.tx_isolation |
+------------------------+
| SERIALIZABLE |
+------------------------+
1 row in set (0.00 sec)

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

A> select * from test ;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+
3 rows in set (0.00 sec)

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

B> update test set num=3 where id=1 ;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

控制超时的参数
innodb_lock_wait_timeout | 120