mysql5.6 innodb_large_prefix引起的一个异常

phenomenon： Specified key was too long; max key length is 3072 bytes

在修改一个数据库字段时，字段容量被限制为了表前缀的大小而不是本身的容量大小

查了一下innodb_large_prefix究竟是什么？

动态行格式DYNAMIC row format 支持最大的索引前缀(3072)。由变量innodb_large_prefix进行控制。

By default, the index key prefix length limit is 767 bytes. See Section 13.1.13, “CREATE INDEX Statement”. For example, you might hit this limit with a column prefix index of more than 255 characters on a TEXT or VARCHAR column, assuming a utf8mb3 character set and the maximum of 3 bytes for each character. When the innodb_large_prefix configuration option is enabled, the index key prefix length limit is raised to 3072 bytes for InnoDB tables that use the DYNAMIC or COMPRESSED row format.

官方上说在utf8mb3（most bytes n）如果设置为 varchar(255)时，索引前缀将大于767，可以扩展为3072，但是实际上 varchar的size可以为65535，这个就限制了整个alter table 的操作

因为建表是时索引没设置大小，默认是超过255的，后面开启了前缀限制，大小会为3072，此时无法做表修改

Reference

https://forums.percona.com/t/utf8mb4-error-1709-hy000-index-column-size-too-large-the-maximum-column-size-is-767-bytes/8336

MySQL8 innodb large prefix

innodb_file_format settings backwards compatible

innodb limits