如何进行MySQL 5.5 隔离级别的测试

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REPEATABLE READ

这是InnoDB默认的隔离级别。对于一致性读，REPEATABLE READ和READ COMMITTED有一个重要的区别：在同一个事务中，所有的一致性读会从这个事务第一次读取的快照中读取数据。如果在相同事务中执行多条相同的SELECT语句，这些SELECT查询语句的返回结果是相同的。对于锁定读（例如SELECT .. FOR UPDATE 或 LOCK IN SHARE MODE），UPDATE, DELETE语句，锁依赖语句是否使用unique索引扫描或范围扫描。对于unique索引扫描，InnoDB只会锁定找到的索引记录。对于其他的搜索条件，InnoDB会锁定范围索引扫描到的记录。

会话①向表插入数据并提交，会话②只有在执行commit或rollback操作后，才可以查询到会话①的插入数据，否则看到的是登录时最初查询到的快照中的数据。

会话①
mysql> show variables like '%iso%';
+---------------+-----------------+
| Variable_name | Value           |
+---------------+-----------------+
| tx_isolation  | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.09 sec)

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

mysql> insert into dept2 values(10,'Research');
Query OK, 1 row affected (0.01 sec)

mysql> commit;
Query OK, 0 rows affected (0.12 sec)

mysql> select * from dept2;
+--------+----------+
| deptno | dname    |
+--------+----------+
|     10 | Research |
+--------+----------+
1 row in set (0.00 sec)

会话②
mysql> show variables like '%iso%';
+---------------+-----------------+
| Variable_name | Value           |
+---------------+-----------------+
| tx_isolation  | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.01 sec)

mysql> select * from dept2;
Empty set (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.16 sec)

mysql> select * from dept2;
+--------+----------+
| deptno | dname    |
+--------+----------+
|     10 | Research |
+--------+----------+
1 row in set (0.00 sec)

会话①
mysql> start transaction
    -> ;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into dept2 values(20,'Maintenance');
Query OK, 1 row affected (0.04 sec)

mysql> commit;
Query OK, 0 rows affected (0.05 sec)

mysql> select * from dept2;
+--------+-------------+
| deptno | dname       |
+--------+-------------+
|     10 | Research    |
|     20 | Maintenance |
+--------+-------------+
2 rows in set (0.00 sec)

会话②
mysql> select * from dept2;
+--------+----------+
| deptno | dname    |
+--------+----------+
|     10 | Research |
+--------+----------+
1 row in set (0.00 sec)

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

mysql> select * from dept2;
+--------+-------------+
| deptno | dname       |
+--------+-------------+
|     10 | Research    |
|     20 | Maintenance |
+--------+-------------+
2 rows in set (0.00 sec)

间隔锁测试，在REPEATABLE-READ隔离模式下，MySQL对数据进行范围、条件扫描时，会对范围内也许并不存在的值进行加锁。

会话①
mysql> select @@global.tx_isolation,@@tx_isolation;
+-----------------------+-----------------+
| @@global.tx_isolation | @@tx_isolation  |
+-----------------------+-----------------+
| REPEATABLE-READ       | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

mysql> select * from dept2;
+--------+-------------+
| deptno | dname       |
+--------+-------------+
|     10 | Research    |
|     20 | Maintenance |
|     30 | Leader      |
+--------+-------------+
3 rows in set (0.00 sec)

mysql> select * from dept2 where deptno < 30 lock in share mode;
+--------+-------------+
| deptno | dname       |
+--------+-------------+
|     10 | Research    |
|     20 | Maintenance |
+--------+-------------+
2 rows in set (0.02 sec)

会话②

mysql> SELECT @@GLOBAL.tx_isolation, @@tx_isolation;
+-----------------------+-----------------+
| @@GLOBAL.tx_isolation | @@tx_isolation  |
+-----------------------+-----------------+
| REPEATABLE-READ       | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

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

mysql> insert into dept2 values(40,'Market');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

READ COMMITTED

类似Oracle的事务隔离级别：每个一致性读，及时在相同的事务中，会读取自己事务最新的快照。对于锁定读（例如SELECT .. FOR UPDATE 或 LOCK IN SHARE MODE），UPDATE, DELETE语句，InnoDB只会锁定索引记录，允许向非锁定的记录插入新的记录。

会话①删除一条数据并执行提交操作后，会话②会立刻查询到会话①更新后的操作。

mysql> SET GLOBAL tx_isolation='READ-COMMITTED';
Query OK, 0 rows affected (0.00 sec)
mysql> exit
Bye

会话①
mysql> use fire
Database changed
mysql> SELECT @@GLOBAL.tx_isolation, @@tx_isolation;
+-----------------------+----------------+
| @@GLOBAL.tx_isolation | @@tx_isolation |
+-----------------------+----------------+
| READ-COMMITTED        | READ-COMMITTED |
+-----------------------+----------------+
1 row in set (0.00 sec)

mysql> select * from dept;
+--------+------------+----------+
| DEPTNO | DNAME      | LOC      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+
4 rows in set (0.08 sec)

mysql> select * from dept2;
+--------+-------------+
| deptno | dname       |
+--------+-------------+
|     10 | Research    |
|     20 | Maintenance |
|     30 | Leader      |
|     40 | Market      |
+--------+-------------+
4 rows in set (0.00 sec)

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

mysql> delete from dept2 where deptno=40;
Query OK, 1 row affected (0.09 sec)

会话②
mysql> use fire
Database changed
mysql> SELECT @@GLOBAL.tx_isolation, @@tx_isolation;
+-----------------------+----------------+
| @@GLOBAL.tx_isolation | @@tx_isolation |
+-----------------------+----------------+
| READ-COMMITTED        | READ-COMMITTED |
+-----------------------+----------------+
1 row in set (0.00 sec)

mysql> select * from dept2;
+--------+-------------+
| deptno | dname       |
+--------+-------------+
|     10 | Research    |
|     20 | Maintenance |
|     30 | Leader      |
|     40 | Market      |
+--------+-------------+
4 rows in set (0.00 sec)

会话①
mysql> commit;
Query OK, 0 rows affected (0.12 sec)

会话②

mysql> select * from dept2;
+--------+-------------+
| deptno | dname       |
+--------+-------------+
|     10 | Research    |
|     20 | Maintenance |
|     30 | Leader      |
+--------+-------------+
3 rows in set (0.00 sec)

间隔锁测试，在READ-COMMITTED隔离模式下，会话②不会受到会话①的影响。

会话①
mysql> select @@global.tx_isolation,@@tx_isolation;
+-----------------------+----------------+
| @@global.tx_isolation | @@tx_isolation |
+-----------------------+----------------+
| READ-COMMITTED        | READ-COMMITTED |
+-----------------------+----------------+
1 row in set (0.00 sec)

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

mysql> select * from dept2 where deptno < 30 lock in share mode;
+--------+-------------+
| deptno | dname       |
+--------+-------------+
|     10 | Research    |
|     20 | Maintenance |
+--------+-------------+
2 rows in set (0.00 sec)

会话②

mysql> SELECT @@GLOBAL.tx_isolation, @@tx_isolation;
+-----------------------+----------------+
| @@GLOBAL.tx_isolation | @@tx_isolation |
+-----------------------+----------------+
| READ-COMMITTED        | READ-COMMITTED |
+-----------------------+----------------+
1 row in set (0.00 sec)

mysql> select * from dept2;
+--------+-------------+
| deptno | dname       |
+--------+-------------+
|     10 | Research    |
|     20 | Maintenance |
|     30 | Leader      |
+--------+-------------+
3 rows in set (0.00 sec)

mysql> insert into dept2 values(40,'Market');
Query OK, 1 row affected (0.11 sec)

READ UNCOMMITTED

SELECT语句允许以非锁定的方式执行，但是只有较早版本的行可以使用。因而，使用这个隔离级别，一些的读操作不是一致性的。这个隔离级别也被称为脏读。

SERIALIZABLE

这个隔离级别类似REPEATABLE READ，但是如果autocommit没有开启的话，InnoDB会隐式将所有SELECT语句转化为SELECT ... LOCK IN SHARE MODE。

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