mysql 事务隔离
Read uncommitted(读取未提交内容)
在该隔离级别，所有的事务都可以看到其他未提交的事务的执行结果。本隔离级别很少用于实际应用，因为它的性能也不比其他的级别好多少。读取未提交的数据，也被称为脏读。
模拟实例： 需要两个终端
#注意，切换隔离级别建议退出mysql终端重新登录，否则有可能修改失败
A终端:
#设置可以隔离模式
mysql> set global tx_isolation="READ-UNCOMMITTED";
Query OK, 0 rows affected (0.00 sec)
mysql> select @@tx_isolation;
+------------------+
| @@tx_isolation |
+------------------+
| READ-UNCOMMITTED |
+------------------+
1 row in set (0.00 sec)
mysql> use wangdk;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> desc user;
+-------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| name | char(20) | NO | | | |
+-------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql> select * from user;
Empty set (0.00 sec)
#开启事务
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
#注意这里并未提交 commit
mysql> insert into user(name) values("test");
Query OK, 1 row affected (0.00 sec)
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)
B终端： mysql> set global tx_isolation="READ-UNCOMMITTED"; Query OK, 0 rows affected (0.00 sec) mysql> select @@tx_isolation; +------------------+ | @@tx_isolation | +------------------+ | READ-UNCOMMITTED | +------------------+ 1 row in set (0.00 sec) mysql> use wangdk; Reading table information for completion of table and column names You can turn off this feature to get a quicker startup with -A Database changed #注意：A终端并未 commit 提交事务。这里已经可以查到这个事务的数据，但是数据随时可以回滚的。 mysql> select * from user; +----+------+ | id | name | +----+------+ | 12 | test | +----+------+ 1 row in set (0.00 sec) #A终端 rollback 后数据消失 mysql> select * from user; Empty set (0.00 sec)
READ-COMMITTED (读取提交内容)
这是大多数数据库系统的默认系统隔离级别（但不是mysql默认的）。它满足了隔离的简单定义：事务的数据只有被提交才能被查询到。这种隔离级别也成为不可重复读，
以为同一事务的其他实例处理期间可能会有新的commit，所以同一个select会有不同的结果。
A终端： mysql> set global tx_isolation="READ-COMMITTED"; Query OK, 0 rows affected (0.00 sec) mysql> select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql> use wangdk; Reading table information for completion of table and column names You can turn off this feature to get a quicker startup with -A Database changed mysql> select * from user; +----+------+ | id | name | +----+------+ | 13 | 1 | | 14 | 2 | | 15 | 4 | +----+------+ 3 rows in set (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) #删除一条数据，未提交事务 mysql> delete from user where id = 15; Query OK, 1 row affected (0.00 sec) #删除后A终端已经发生变化 mysql> select * from user; +----+------+ | id | name | +----+------+ | 13 | 1 | | 14 | 2 | +----+------+ 2 rows in set (0.00 sec)
B终端：
mysql> set global tx_isolation="READ-COMMITTED"; Query OK, 0 rows affected (0.00 sec) mysql> select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql> use wangdk; Database changed mysql> select * from user; +----+------+ | id | name | +----+------+ | 13 | 1 | | 14 | 2 | | 15 | 4 | +----+------+ 3 rows in set (0.00 sec) #事务未提交B终端未发生变化 mysql> select * from user; +----+------+ | id | name | +----+------+ | 13 | 1 | | 14 | 2 | | 15 | 4 | +----+------+ 3 rows in set (0.00 sec)
Repeatable READ (可以重复读)
这是Mysql默认的事务隔离级别，它确保同一事务的多个实例在并发读数据时候，会看到同样的数据行。不过理论上，这会导致另一个问题（幻读）
（Phantom Read）简单的说，幻读是指永固读取某一个范围的数据行时，另一个事务又在该范围内插入了新行，当用户在读取该范围的
数据行是，会发现有新的幻影行，Innodb和falcon存储引擎通过多版本并发控制解决该问题 （MVCC，Multiversion Concurrency Control）机制
理解：A begin 读取的数据 和 B begin 读取的数据相同，如果B插入了数据，并commit ，A等待自己的事务完毕后才能重新读取B事务提交的数据。
模拟：
A终端：
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
B终端
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)
mysql> select * from user;
+----+--------+
| id | name |
+----+--------+
| 21 | wangdk |
+----+--------+
1 row in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into user(name) values("hello");
Query OK, 1 row affected (0.01 sec)
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from user;
+----+--------+
| id | name |
+----+--------+
| 21 | wangdk |
| 22 | hello |
+----+--------+
2 rows in set (0.00 sec)
A终端：
#并未查找到B事务的数据
mysql> select * from user;
+----+--------+
| id | name |
+----+--------+
| 21 | wangdk |
+----+--------+
1 row in set (0.00 sec)
mysql> commit;
#必须等待A事务完成，才能读取到B事务已经提交上来的数据
mysql> select * from user;
+----+--------+
| id | name |
+----+--------+
| 21 | wangdk |
| 22 | hello |
+----+--------+
3 rows in set (0.00 sec)
Serializable（可串行化）
这种最高的隔离级别，它通过强制事务排序，使之不能相互冲突。从而解决幻读的问题。简而言之，它是在每个读的数据行上加上共享锁
在这个级别，可能导致大量的超时和锁竞争
实例： A终端： mysql> select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | SERIALIZABLE | +----------------+ 1 row in set (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from wangdk.user; +----+--------+ | id | name | +----+--------+ | 21 | wangdk | | 22 | hello | | 23 | hello | +----+--------+ 3 rows in set (0.00 sec) #先不要提交，要先操作B终端，最后一句在执行commit mysql> commit; Query OK, 0 rows affected (0.00 sec) B 终端： mysql> select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | SERIALIZABLE | +----------------+ 1 row in set (0.01 sec) mysql> select * from wangdk.user; +----+--------+ | id | name | +----+--------+ | 21 | wangdk | | 22 | hello | | 23 | hello | +----+--------+ 3 rows in set (0.00 sec) #看到了么，更新秒数，等待A终端commit以后，这一句才会执行 mysql> update wangdk.user set name="wangdk1" where id = 21; Query OK, 1 row affected (17.09 sec) Rows matched: 1 Changed: 1 Warnings: 0