在MySQL中设置事务隔离级别有2种方法:
1 在my.cnf中设置，在mysqld选项中如下设置
[mysqld]
transaction-isolation = READ-COMMITTED
2 在mysql窗口用set命令重置
- mysql> set global tx_isolation='REPEATABLE-READ';
Query OK, 0 rows affected (0.01 sec)
mysql>
查询当前的会话事务级别，可以使用：
- mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set (0.00 sec)
mysql>
查询全局的事务隔离级别，可以使用
- mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| READ-COMMITTED |
+-----------------------+
1 row in set (0.00 sec)
mysql>
在Serializable模式下。
- mysql> system cat /usr/local/mysql56m2/my.cnf |grep transaction-isolation
transaction-isolation = READ-COMMITTED
mysql>
复制二进制与隔离级别的关系
在SERIALIZABLE模式下，Innodb存储引擎会对每个select语句自动加Lock in sharedmode，给每一个读操作加共享锁。因此在这个隔离级别下，读占用锁了，一致性的非锁定读不再予以支持。因为Innodb存储引擎在 repeatable read 模式下就已经达到了3度的隔离，所以一般不在本地事务中使用serializable隔离级别，serializable的事务隔离级别主要用于 innodb存储引擎的分布式事务。
在Read committed的隔离模式下，除了唯一性约束检查以及外键约束检查需要Gap lock，innodb存储引擎不会使用gap lock的锁算法。不过使用read committed隔离级别需要注意一些问题，mysql5.1中，Read committed的事务隔离级别默认只能在replication的二进制为row格式下，如果二进制默认在statement模式下，则会报如下错 误：
- mysql> select @@version;
+-------------+
| @@version |
+-------------+
| 5.5.25a-log |
+-------------+
1 row in set (0.00 sec)
mysql>
mysql> select @@binlog_format;
+-----------------+
| @@binlog_format |
+-----------------+
| STATEMENT |
+-----------------+
1 row in set (0.00 sec)
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+
1 row in set (0.00 sec)
mysql> set tx_isolation='READ-COMMITTED';
Query OK, 0 rows affected (0.00 sec)
mysql> use test;
Database changed
mysql> create table a (b int, primary key (b)) engine=innodb;
ERROR 1050 (42S01): Table 'a' already exists
mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set (0.00 sec)
mysql> begin
-> ;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into a select 100000;
ERROR 1665 (HY000): Cannotexecute statement: impossible to write to binary log since BINLOG_FORMAT =STATEMENT and at least one table uses a storage engine limited to row-basedlogging. InnoDB is limited to row-logging when transaction isolation level isREAD COMMITTED or READ UNCOMMITTED.
ERROR 1665 (HY000):
Cannotexecute statement: impossible to write to binary log since
BINLOG_FORMAT =STATEMENT and at least one table uses a storage engine
limited to row-basedlogging. InnoDB is limited to
row-logging when transaction isolation level isREAD COMMITTED or READ
UNCOMMITTED.
[Note]：在mysql5.1以及mysql5.6模式下实验过都是如此。也许可以知道通过将innodb_locks_unsafe_for_binlog设置为1，来可以使binlog日志在statement下使用readcommitted的事务隔离级别：
- mysql> select @@innodb_locks_unsafe_for_binlog;
+----------------------------------+
| @@innodb_locks_unsafe_for_binlog |
+----------------------------------+
| 0 |
+----------------------------------+
1 row in set (0.00 sec)
mysql> set global innodb_locks_unsafe_for_binlog=1;
ERROR 1238 (HY000): Variable 'innodb_locks_unsafe_for_binlog' is a readonly variable
mysql>
此参数是只读模式，需要修改my.cnf重新启动才行。
在my.cnf里面的[mysqld]添加
[mysqld]
innodb_locks_unsafe_for_binlog = 1
然后重启，然后去check模仿一个事务操作，如下所示：
- mysql> select @@innodb_locks_unsafe_for_binlog;
+----------------------------------+
| @@innodb_locks_unsafe_for_binlog |
+----------------------------------+
| 1 |
+----------------------------------+
1 row in set (0.00 sec)
mysql>
mysql> use test;
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 @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set (0.00 sec)
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into t select 15;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> commit;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from t;
+--------+
| id |
+--------+
| 1 |
| 12 |
| 15 |
| 11111 |
| 111110 |
+--------+
5 rows in set (0.00 sec)