MySQL的自动提交模式

 

 

默认情况下, MySQL启用自动提交模式（变量autocommit为ON）。这意味着, 只要你执行DML操作的语句,MySQL会立即隐式提交事务（Implicit Commit）。这个跟SQL Server基本是类似的。如果你了解SQL Server数据库的话。

 

 

 

查看autocommit模式

 

 

由于变量autocommit分会话系统变量与全局系统变量，所以查询的时候，最好区别是会话系统变量还是全局系统变量。

 

 

mysql> show session variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set (0.00 sec)

mysql> show global variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set (0.00 sec)

mysql>

 

 

 

Value的值为ON，表示autocommit开启。OFF表示autocommit关闭。

 

 

 

修改autocommit模式

 

 

 

 

mysql> set session autocommit=0;
Query OK, 0 rows affected (0.00 sec)

mysql> show session variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set (0.00 sec)

mysql> show global variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set (0.00 sec)

mysql>

mysql> set global autocommit=0;
Query OK, 0 rows affected (0.00 sec)

mysql> show global variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set (0.01 sec)

mysql>

 

 

 

注意，上述SQL修改会话系统变量或全局系统变量，只对当前实例有效，如果MySQL服务重启的话，这些设置就会丢失，如果要永久生效，就必须在配置文件中修改系统变量。

 

 

[mysqld]

autocommit=0

 

 

 

不过网上还有种方式，如下所示，我在MySQL 5.6/5.7下测试，发现不生效，查了一下，这种方式似乎从MySQL 5.6开始已经不生效了，必须用autocommit=0这种方式替换。

 

[mysqld]

init_connect='SET autocommit=0'

 

 

 

 

autocommit与显性事务的关系

 

 

对于显性事务start transaction或begin， 在自动提交模式关闭（关闭隐式提交）的情况下，开启一个事务上下文。首先数据库会隐式提交之前的还未被提交的操作，同时开启一个新事务。如有不明，可以用下面小实验理解一下：

 

测试如下所示：

 

mysql> select connection_id();
+-----------------+
| connection_id() |
+-----------------+
|               1 |
+-----------------+
1 row in set (0.00 sec)
mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set (0.00 sec)

mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 sec)
mysql> delete from MyDB.test where name='kerry';
Query OK, 1 row affected (0.00 sec)

 

 

此时在会话2中查看，此时可以查询到会话ID为1的事务信息， 如下所示

 

mysql>  select connection_id();
+-----------------+
| connection_id() |
+-----------------+
|               2 |
+-----------------+
1 row in set (0.00 sec)

mysql> SELECT a.trx_state,
    ->        b.event_name,
    ->        a.trx_started,
    ->        b.timer_wait / 1000000000000 timer_wait,
    ->        a.trx_mysql_thread_id        blocking_trx_id,
    ->        b.sql_text
    -> FROM   information_schema.innodb_trx a,
    ->        performance_schema.events_statements_current b,
    ->        performance_schema.threads c
    -> WHERE  a.trx_mysql_thread_id = c.processlist_id
    ->        AND b.thread_id = c.thread_id;
+-----------+----------------------+---------------------+------------+-----------------+------------------------------------------+
| trx_state | event_name           | trx_started         | timer_wait | blocking_trx_id | sql_text                                 |
+-----------+----------------------+---------------------+------------+-----------------+------------------------------------------+
| RUNNING   | statement/sql/delete | 2018-03-23 14:55:00 |     0.0010 |               1 | delete from MyDB.test where name='kerry' |
+-----------+----------------------+---------------------+------------+-----------------+------------------------------------------+
1 row in set (0.00 sec)

 

如果在会话1当中开启显性事务，那么之前挂起的事务会自动提交，然后，你再去会话2当中查询，就发现之前的DELETE操作已经提交。

 

 

 

 

mysql>  select connection_id();
+-----------------+
| connection_id() |
+-----------------+
|               2 |
+-----------------+
1 row in set (0.00 sec)

mysql> SELECT a.trx_state,
    ->        b.event_name,
    ->        a.trx_started,
    ->        b.timer_wait / 1000000000000 timer_wait,
    ->        a.trx_mysql_thread_id        blocking_trx_id,
    ->        b.sql_text
    -> FROM   information_schema.innodb_trx a,
    ->        performance_schema.events_statements_current b,
    ->        performance_schema.threads c
    -> WHERE  a.trx_mysql_thread_id = c.processlist_id
    ->        AND b.thread_id = c.thread_id;
+-----------+----------------------+---------------------+------------+-----------------+------------------------------------------+
| trx_state | event_name           | trx_started         | timer_wait | blocking_trx_id | sql_text                                 |
+-----------+----------------------+---------------------+------------+-----------------+------------------------------------------+
| RUNNING   | statement/sql/delete | 2018-03-23 14:55:00 |     0.0010 |               1 | delete from MyDB.test where name='kerry' |
+-----------+----------------------+---------------------+------------+-----------------+------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT a.trx_state,
    ->        b.event_name,
    ->        a.trx_started,
    ->        b.timer_wait / 1000000000000 timer_wait,
    ->        a.trx_mysql_thread_id        blocking_trx_id,
    ->        b.sql_text
    -> FROM   information_schema.innodb_trx a,
    ->        performance_schema.events_statements_current b,
    ->        performance_schema.threads c
    -> WHERE  a.trx_mysql_thread_id = c.processlist_id
    ->        AND b.thread_id = c.thread_id;
Empty set (0.00 sec)

 

 

With START TRANSACTION, autocommit remains disabled until you end the transaction with COMMIT or ROLLBACK. The autocommit mode then reverts to its previous state

使用START TRANSACTION，自动提交将保持禁用状态，直到你使用COMMIT或ROLLBACK结束事务。 自动提交模式然后恢复到之前的状态（如果start transaction 前 autocommit = 1，则完成本次事务后 autocommit 还是 1。如果 start transaction 前 autocommit = 0，则完成本次事务后 autocommit 还是 0）

 

 

 

 

参考资料：

 

https://dev.mysql.com/doc/refman/5.7/en/commit.html