在MySQL中設置事務隔離級別有2種方法:
1 在my.cnf中設置，在mysqld選項中如下設置
[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>
查詢當前的會話事務級別，可以使用：
- mysql> select @@tx_isolation;
- +----------------+
- | @@tx_isolation |
- +----------------+
- | READ-COMMITTED |
- +----------------+
- 1 row in set (0.00 sec)
- mysql>
查詢全局的事務隔離級別，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>
復制二進制與隔離級別的關系
在SERIALIZABLE模式下，Innodb存儲引擎會對每個select語句自動加Lock in sharedmode，給每一個讀操作加共享鎖。因此在這個隔離級別下，讀占用鎖了，一致性的非鎖定讀不再予以支持。因為Innodb存儲引擎在 repeatable read 模式下就已經達到了3度的隔離，所以一般不在本地事務中使用serializable隔離級別，serializable的事務隔離級別主要用於 innodb存儲引擎的分布式事務。
在Read committed的隔離模式下，除了唯一性約束檢查以及外鍵約束檢查需要Gap lock，innodb存儲引擎不會使用gap lock的鎖算法。不過使用read committed隔離級別需要注意一些問題，mysql5.1中，Read committed的事務隔離級別默認只能在replication的二進制為row格式下，如果二進制默認在statement模式下，則會報如下錯 誤：
- 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模式下實驗過都是如此。也許可以知道通過將innodb_locks_unsafe_for_binlog設置為1，來可以使binlog日志在statement下使用readcommitted的事務隔離級別：
- 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>
此參數是只讀模式，需要修改my.cnf重新啟動才行。
在my.cnf里面的[mysqld]添加
[mysqld]
innodb_locks_unsafe_for_binlog = 1
然后重啟，然后去check模仿一個事務操作，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)