朋友发过来一个SQL,，让我执行以下：

CREATE TABLE `ttt` ( `id` DOUBLE , `select_type` VARCHAR (57), `table` VARCHAR (192), `type` VARCHAR (30), `possible_keys` VARCHAR (22288), `key` VARCHAR (192), `key_len` VARCHAR (22288), `ref` VARCHAR (3072), `rows` DOUBLE , `Extra` VARCHAR (765)); INSERT INTO `ttt` (`id`, `select_type`, `table`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `Extra`) VALUES('1','PRIMARY','aaa','index',NULL,'email_2','51',NULL,'5','Using where; Using index');INSERT INTO `ttt` (`id`, `select_type`, `table`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `Extra`) VALUES('2','DEPENDENT SUBQUERY','bbb','ref','id','id','5','test.aaa.id','1','Using where; Using index');

执行第一句建表的时候就卡住了，报错如下：

mysql> CREATE TABLE `ttt` (    -> `id` DOUBLE ,    -> `select_type` VARCHAR (57),    -> `table` VARCHAR (192),    -> `type` VARCHAR (30),    -> `possible_keys` VARCHAR (12288),    -> `key` VARCHAR (192),    -> `key_len` VARCHAR (12288),    -> `ref` VARCHAR (3072),    -> `rows` DOUBLE ,    -> `Extra` VARCHAR (765)    -> ); ERROR 1118 (42000): Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. This includes storage overhead, check the manual. You have to change some columns to TEXT or BLOBsmysql>

看了提示，表的2个varchar字段长度设置过长了，需要改成text，blob之类的类型，修改之后执行成功了：

mysql> use test;Database changedmysql> mysql> CREATE TABLE `ttt` (    -> `id` DOUBLE ,    -> `select_type` VARCHAR (57),    -> `table` VARCHAR (192),    -> `type` VARCHAR (30),    -> `possible_keys` TEXT,    -> `key` VARCHAR (192),    -> `key_len`  TEXT,    -> `ref` VARCHAR (3072),    -> `rows` DOUBLE ,    -> `Extra` VARCHAR (765)    -> ); Query OK, 0 rows affected (0.00 sec)mysql> INSERT INTO `ttt` (`id`, `select_type`, `table`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `Extra`) VALUES('1','PRIMARY','aaa','index',NULL,'email_2','51',NULL,'5','Using where; Using index');Query OK, 1 row affected (0.04 sec)mysql> INSERT INTO `ttt` (`id`, `select_type`, `table`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `Extra`) VALUES('2','DEPENDENT SUBQUERY','bbb','ref','id','id','5','test.aaa.id','1','Using where; Using index');Query OK, 1 row affected (0.00 sec)mysql> select * from ttt;+------+--------------------+-------+-------+---------------+---------+---------+-------------+------+--------------------------+| id   | select_type        | table | type  | possible_keys | key     | key_len | ref         | rows | Extra                    |+------+--------------------+-------+-------+---------------+---------+---------+-------------+------+--------------------------+|    1 | PRIMARY            | aaa   | index | NULL          | email_2 | 51      | NULL        |    5 | Using where; Using index ||    2 | DEPENDENT SUBQUERY | bbb   | ref   | id            | id      | 5       | test.aaa.id |    1 | Using where; Using index |+------+--------------------+-------+-------+---------------+---------+---------+-------------+------+--------------------------+2 rows in set (0.00 sec)

疑惑：varchar(N)，这个N不是最大为65535吗？为什么设置成12288就会报错？12288比65535小很多啊。

有问题上官网：http://dev.mysql.com/doc/refman/5.6/en/char.html

In contrast to CHAR, VARCHAR values are stored as a 1-byte or 2-byte length prefix plus data. The length prefix indicates the number of bytes in the value. A column uses one length byte if values require no more than 255 bytes, two length bytes if values may require more than 255 bytes.

原来有N有255这样一个坎啊，超过了N的意义就不一样了。 

后续测试1 ，朋友木木说是字符集的问题，那么我就执行之前设置下字符集，不过还是报错，但是我在SQLyog工具的窗口里面执行是成功（工具里面自动转变成mediumtext类型了）。

mysql> set names utf8;Query OK, 0 rows affected (0.00 sec)mysql> DROP TABLE ttt;ERROR 1051 (42S02): Unknown table 'test.ttt'mysql> CREATE TABLE `ttt` (    -> `id` DOUBLE ,    -> `select_type` VARCHAR (57),    -> `table` VARCHAR (192),    -> `type` VARCHAR (65535),    -> `key` VARCHAR(20),    -> `possible_keys` VARCHAR (100),    -> `key_len`  VARCHAR (65535),    -> `ref` VARCHAR (11),    -> `rows` DOUBLE ,    -> `Extra` VARCHAR (765)    -> ); ERROR 1074 (42000): Column length too big for column 'type' (max = 21845); use BLOB or TEXT insteadmysql> set names gbk;Query OK, 0 rows affected (0.00 sec)mysql> DROP TABLE ttt;ERROR 1051 (42S02): Unknown table 'test.ttt'mysql> CREATE TABLE `ttt` (    -> `id` DOUBLE ,    -> `select_type` VARCHAR (57),    -> `table` VARCHAR (192),    -> `type` VARCHAR (65535),    -> `key` VARCHAR(20),    -> `possible_keys` VARCHAR (100),    -> `key_len`  VARCHAR (65535),    -> `ref` VARCHAR (11),    -> `rows` DOUBLE ,    -> `Extra` VARCHAR (765)    -> ); ERROR 1074 (42000): Column length too big for column 'type' (max = 21845); use BLOB or TEXT insteadmysql>

最后总结：这个可能是超过 一个表 关于 非十六进制字段 64k的限制了。

上官网：http://dev.mysql.com/doc/refman/5.5/en/column-count-limit.html：

Every table (regardless of storage engine) has a maximum row size of65,535 bytes. Storage engines may place additional constraints on this limit, reducing the effective maximum row size.

65,535所说明的是针对的是整个表的非大字段类型的字段的bytes总合。