Specified key was too long; max key length is 767 bytes

mysql创建表时报错：

Specified key was too long; max key length is 767 bytes

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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)