MySQL事务学习-->隔离级别

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
[sql] 
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：
[sql] 
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
[sql] 
mysql> select @@global.tx_isolation;  
+-----------------------+  
| @@global.tx_isolation |  
+-----------------------+  
| READ-COMMITTED        |  
+-----------------------+  
1 row in set (0.00 sec)  
    
mysql>  
 
 
 
在Serializable模式下。
[sql] 
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：
[sql] 
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：
[sql] 
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：
[sql] 
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)