MySQL8.0 查看默认事务的隔离级别，并模拟测试
文章目录
- 3. MySQL8.0 查看默认事务的隔离级别，并测试
- 3.1 更改隔离级别为：未提交读测试
- 3.2 测试隔离级别为：提交读
- 3.3 测试可重复读
- 3.3.1 模拟幻读
- 3.4 可串行化就不测试了，就是顺序的问题，谁的事务先开启谁进行，其它的都要等待
3. MySQL8.0 查看默认事务的隔离级别，并测试
如下：MySQL默认是：可重复读
mysql> select @@global.transaction_isolation,@@transaction_isolation;
+--------------------------------+-------------------------+
| @@global.transaction_isolation | @@transaction_isolation |
+--------------------------------+-------------------------+
| REPEATABLE-READ | REPEATABLE-READ |
+--------------------------------+-------------------------+
1 row in set (0.00 sec)
3.1 更改隔离级别为：未提交读测试
两个命令行客户端分别为A，B；不断改变A的隔离级别，在B端修改数据。
但是不改变B的隔离级别，只改变A的,B还是默认的隔离级别：可重复读
A:
mysql> set session transaction isolation level read uncommitted;
Query OK, 0 rows affected (0.00 sec)
## 查看
mysql> select @@global.transaction_isolation,@@transaction_isolation;
+--------------------------------+-------------------------+
| @@global.transaction_isolation | @@transaction_isolation |
+--------------------------------+-------------------------+
| REPEATABLE-READ | READ-UNCOMMITTED |
+--------------------------------+-------------------------+
1 row in set (0.00 sec)
- 准一张表,测试事务
CREATE TABLE `test` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`num` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;
## 插入3条数据
INSERT INTO `trans`.`test` (`id`, `num`) VALUES ('1', '1');
INSERT INTO `trans`.`test` (`id`, `num`) VALUES ('2', '2');
INSERT INTO `trans`.`test` (`id`, `num`) VALUES ('3', '3');
用两个窗口分别打开MySQL
mysql -uroot -p123456
- 窗口1: 启动事务A，数据库3条记录是初始状态
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
- 窗口2. 启动事务B ，修改数据，但是不提交事务B
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set num =33 where id =3;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
## 在事务B内已经修改了
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 2 |
| 3 | 33 |
+----+-----+
3 rows in set (0.00 sec)
- 窗口1 事务A 查看数据是否已经被修改
发现已经被修改，这就是所谓的“脏读”
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 2 |
| 3 | 33 |
+----+-----+
3 rows in set (0.00 sec)
- 窗口2 回滚事务B
mysql> rollback ;
Query OK, 0 rows affected (0.04 sec)
- 窗口1 事务A查看数据是否回滚
发现已经回滚
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
- 经过上面的实验可以得出结论，事务B更新了一条记录，但是没有提交，此时事务A可以查询出未提交记录。造成脏读现象。未提交读是最低的隔离级别
3.2 测试隔离级别为：提交读
把窗口A的隔离级别换为：提交读，窗口B的不换还是默认的
mysql> set session transaction isolation level read committed ;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@global.transaction_isolation,@@transaction_isolation;
+--------------------------------+-------------------------+
| @@global.transaction_isolation | @@transaction_isolation |
+--------------------------------+-------------------------+
| REPEATABLE-READ | READ-COMMITTED |
+--------------------------------+-------------------------+
1 row in set (0.00 sec)
- 窗口A开启事务，查询
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
- 窗口2开启事务，修改数据，并查看
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> update test set num =22 where id =2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 22 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
- 窗口A再次查看数据有没有被更新
发现没有，隔离级别生效
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
- 窗口B 提交事务
mysql> commit ;
Query OK, 0 rows affected (0.08 sec)
- 窗口A 再次查询数据
发现数据被修改了，符合预期
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 22 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec
看是正常的代码，但是这个时候有什么问题？
有：在事务A中，两次读取的数据不一致，即不可重复读
- 经过上面的实验可以得出结论，已提交读隔离级别解决了脏读的问题，但是出现了不可重复读的问题，即事务A在两次查询的数据不一致，因为在两次查询之间事务B更新了一条数据。已提交读只允许读取已提交的记录，但不要求可重复读。
3.3 测试可重复读
- 事务A 设置可重读，并开启事务查看
mysql> set session transaction isolation level repeatable read ;
Query OK, 0 rows affected (0.00 sec)
mysql> select @@global.transaction_isolation,@@transaction_isolation;
+--------------------------------+-------------------------+
| @@global.transaction_isolation | @@transaction_isolation |
+--------------------------------+-------------------------+
| REPEATABLE-READ | REPEATABLE-READ |
+--------------------------------+-------------------------+
1 row in set (0.00 sec)
## 查看数据
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 22 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
- 事务B查看事务，默认的即可
mysql> select @@global.transaction_isolation,@@transaction_isolation;
+--------------------------------+-------------------------+
| @@global.transaction_isolation | @@transaction_isolation |
+--------------------------------+-------------------------+
| REPEATABLE-READ | REPEATABLE-READ |
+--------------------------------+-------------------------+
1 row in set (0.00 sec)
## 事务B开启事务并修改id=1的数据
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 22 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
mysql> update test set num =11 where id =1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 11 |
| 2 | 22 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
- 窗口A 查看发现没有被修改，符合预期
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 22 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
- 窗口B提交事务并查看，发现数据已经被更新了
mysql> commit ;
Query OK, 0 rows affected (0.08 sec)
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 11 |
| 2 | 22 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
- 窗口A查看数据
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 22 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec)
发现数据依然没有被修改，符合可重复读的预期，即在一个事务内读取一个值的数据始终应该保持一致
但是！！！
有什么问题没有！！！——————》》数据不是最新的数据
- 如果我 再次开启事务并提交事务在窗口B插入一条记录呢？
下面在B窗口中，但是不开启事务
mysql> start transaction;
mysql> insert into test (num) value(4);
Query OK, 1 row affected (0.07 sec)
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 11 |
| 2 | 22 |
| 3 | 3 |
| 4 | 4 |
+----+-----+
4 rows in set (0.00 sec)
- 窗口A 再次查看，发现还是没有获取到最新数据
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 1 |
| 2 | 22 |
| 3 | 3 |
+----+-----+
3 rows in set (0.00 sec
- 把事务A提交了，获取数据，才发现是最新的
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test ;
+----+-----+
| id | num |
+----+-----+
| 1 | 11 |
| 2 | 22 |
| 3 | 3 |
| 4 | 4 |
+----+-----+
4 rows in set (0.00 sec
由以上的实验可以得出结论，可重复读隔离级别只允许读取已提交记录，而且在一个事务A两次读取一个记录期间，其他事务B如果更新该记录，事务A是看不到的。但该事务不要求与其他事务可串行化。例如，当一个事务可以找到由一个已提交事务更新的记录，但是可能产生幻读问题(注意是可能，因为数据库对隔离级别的实现有所差别)。像以上的实验，就没有出现数据幻读的问题。
3.3.1 模拟幻读
上述的实验没有出现幻读的现象，下面我们来模拟一下什么是幻读？
- 保证两个窗口AB的隔离级别：都是可重复读
select @@global.transaction_isolation,@@transaction_isolation;
+--------------------------------+-------------------------+
| @@global.transaction_isolation | @@transaction_isolation |
+--------------------------------+-------------------------+
| REPEATABLE-READ | REPEATABLE-READ |
+--------------------------------+-------------------------+
## 假设现在已经有了6条记录分别是
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 11 |
| 2 | 22 |
| 3 | 3 |
| 4 | 4 |
| 5 | 55 |
| 6 | 55 |
+----+-----+
6 rows in set (0.00 sec)
- 在A窗口开启事务，查询
mysql> start transaction ;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 11 |
| 2 | 22 |
| 3 | 3 |
| 4 | 4 |
| 5 | 55 |
| 6 | 55 |
+----+-----+
6 rows in set (0.00 sec)
- 在B窗口不开启事务直接插入一条记录，num=7
mysql> insert into test (num) values (7);
Query OK, 1 row affected (0.10 sec)
- 在A窗口查询，是否有num=7的记录
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 11 |
| 2 | 22 |
| 3 | 3 |
| 4 | 4 |
| 5 | 55 |
| 6 | 55 |
+----+-----+
6 rows in set (0.00 sec)
发现没有Num=7的记录，验证了可重复读的特性，符合预期，但是我们怎么模拟幻读呢？
- 在窗口A中添加一条记录，num也等于7,并查询
mysql> insert into test (num) values (7);
Query OK, 1 row affected (0.00 sec)
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 11 |
| 2 | 22 |
| 3 | 3 |
| 4 | 4 |
| 5 | 55 |
| 6 | 55 |
| 8 | 7 |
+----+-----+
7 rows in set (0.00 sec)
发现直接跳过了id=7的数据，也说明了肯定有其它的线程在操作，试想我们如果更新num=7的记录会更新几条呢？如果更新了1条就不是幻读，如果更新了2条就产生了幻读。下面验证一下
- 在窗口A中更新num=7的记录，并再次查询
mysql> update test set num =777 where num =7;
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0
mysql> select * from test;
+----+-----+
| id | num |
+----+-----+
| 1 | 11 |
| 2 | 22 |
| 3 | 3 |
| 4 | 4 |
| 5 | 55 |
| 6 | 55 |
| 7 | 777 |
| 8 | 777 |
+----+-----+
8 rows in set (0.00 sec)
哇！！！ 结果匹配了两条，而且查询出来多了一条记录，这难道是幻读？？？？YES ，这就是幻读
模拟成功，类似删除也有类似效果，我们不再模拟
大致思路是：在A窗口开启事务，此时查询到两条777的记录，在B窗口新添加一条记录num =777,再加上原来两条777的记录公有三条，此时在A窗口执行删除语句
mysql> delete from test where num =777;
Query OK, 3 rows affected (0.00 sec)
发现删除了3条，但是预期是2条，所以出现了幻读