MySQL has a prefix limitation of 767 bytes in InnoDB, and 1000 bytes in MyISAM. This has never been a problem for me, until I started using UTF-16 as the character set for one of my databases. UTF-16 can use up to 4 bytes per character which means that in an InnoDB table, you can’t have any keys longer than 191 characters. Take this
CREATE statement for example:
CREATE TABLE `user` ( `id` int(11) NOT NULL AUTO_INCREMENT, `username` varchar(32) NOT NULL, `password` varchar(64) NOT NULL, `email` varchar(255) NOT NULL, PRIMARY KEY (`id`), UNIQUE KEY `UNIQ_8D93D649F85E0677` (`username`), UNIQUE KEY `UNIQ_8D93D649E7927C74` (`email`) ) ENGINE=InnoDB DEFAULT CHARSET=utf16 AUTO_INCREMENT=1 ;
This will fail with an error like
Specified key was too long; max key length is 767 bytes, because the
UNIQUE INDEX on the email field requires at least 1020 bytes (255 * 4).
Unfortunately there is no real solution to this. Your only options are to either reduce the size of the column, use a different character set (like UTF-8), or use a different engine (like MyISAM). In this case I switched the character set to UTF-8 which raised the maximum key length to 255 characters.