MySQL create table 建表错误Specified key was too long; max key length is 727 bytes

MySQL 创建表异常：

Specified key was too long; max key length is 727 bytes

原因是字段设置长度限制了767个字节 ，一个varchar占用4个字节，

解决办法:

设置varchar字符长度不能超过191 （191*4=764）

或者启用innodb_large_prefix，限制值会增加到3072

mysql> set global innodb_file_format = BARRACUDA;
Query OK, 0 rows affected (0.00 sec)
 
mysql> set global innodb_large_prefix = ON;
Query OK, 0 rows affected (0.00 sec)