ERROR 2020 (HY000): Got packet bigger than 'max_allowed_packet' bytes

今天在測試環境發生了貌似怪異的報錯，

表結構如下：

CREATE TABLE `test` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `step` bigint(20) NOT NULL COMMENT 'step',
  `data` longtext COMMENT 'step data',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1778606 DEFAULT CHARSET=utf8 COMMENT='test'           

要查詢一條資料

select char_length(data) from test where id=1;           

結果卻是：

mysql> ERROR 2020 (HY000): Got packet bigger than 'max_allowed_packet' bytes           

而實際情況是：

mysql> select length(data),char_length(data) from test where id=1;
+--------------+-------------------+
| length(data) | char_length(data) |
+--------------+-------------------+
|     22270339 |          22270339 |
+--------------+-------------------+
1 row in set (0.12 sec)           

為什麼已經把

max_allowed_packet

設定成最大了，還是報ERROR 2020 (HY000)錯啊？

mysql> show global variables like "max_allowed_packet";
+--------------------+------------+
| Variable_name      | Value      |
+--------------------+------------+
| max_allowed_packet | 1073741824 |
+--------------------+------------+
1 row in set (0.00 sec)           

即使裡面存的中文utf8占3個位元組，22270339*3<1073741824啊，忽然靈機一動，看了

mysql> show global variables like "innodb_buffer_pool_size";
+-------------------------+-----------+
| Variable_name           | Value     |
+-------------------------+-----------+
| innodb_buffer_pool_size | 880803840 |
+-------------------------+-----------+
1 row in set (0.00 sec)

mysql> select 880803840/(1024*1024*1024.0);                                                                                                                                               +------------------------------+
| 880803840/(1024*1024*1024.0) |
+------------------------------+
|                       0.8203 |
+------------------------------+
1 row in set (0.00 sec)           

max_allowed_packet已經大于innodb_buffer_pool_size了，懷疑是規格太小導緻，等更新下執行個體規格看看