mysql> show VARIABLES like 'max_allowed_packet';
+--------------------+------------+
| Variable_name      | Value      |
+--------------------+------------+
| max_allowed_packet | 1073741824 |
+--------------------+------------+
1 row in set
max_allowed_packet的单位为字节:
-- 转化为Mb,就是1024Mb
mysql> select 1073741824/1024/1024;
+----------------------+
| 1073741824/1024/1024 |
+----------------------+
| 1024.00000000        |
+----------------------+
1 row in set
修改方法1-配置文件修改
可以编辑my.cnf,在[mysqld]段或者mysql的server配置段进行修改。
max_allowed_packet = 20M
修改my.cnf,配置要重载才能生效
修改方法2-命令修改
参数生效范围为global,不是session. 如果服务器重启设置会失效
set global max_allowed_packet = 2*1024*1024*10
查询时使用的是show variables的话，发现设置好像并没有生效，这是因为show variables等同于show session variables，查询的是会话变量，只有使用show global variables，查询的才是全局变量。
查询 max_allowed_packet = 1073741824
mysql> show VARIABLES like 'max_allowed_packet';
+--------------------+------------+
| Variable_name      | Value      |
+--------------------+------------+
| max_allowed_packet | 1073741824 |
+--------------------+------------+
1 row in set
设置 max_allowed_packet = 1048576
mysql> set global max_allowed_packet = 1*1024*1024;
Query OK, 0 rows affected
查询结果没变
mysql> show VARIABLES like 'max_allowed_packet';
+--------------------+------------+
| Variable_name      | Value      |
+--------------------+------------+
| max_allowed_packet | 1073741824 |
+--------------------+------------+
1 row in set
mysql> show global VARIABLES like 'max_allowed_packet';
+--------------------+---------+
| Variable_name      | Value   |
+--------------------+---------+
| max_allowed_packet | 1048576 |
+--------------------+---------+
1 row in set
max_allowed_packet 值设置过小将导致单个记录超过限制后写入数据库失败，且后续记录写入也将失败，为了数据完整性，需要考虑到事务因素。
最后编辑于：2017-12-08 00:57