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

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:

  `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`)

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.

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  • Tad Coffin

    Couldn’t you use an index prefix length?:

    Indexes can be created that use only the leading part of column values, using col_name(length) syntax to specify an index prefix length:

    Prefixes can be specified for CHAR, VARCHAR, BINARY, and VARBINARY columns.

    • Joseph

      Hi Tad,

      Perhaps I don’t understand MySQL’s index prefixing, but it seems to me that a prefix might not be the best solution for a UNIQUE index? Would an index prefix which is 14 characters in length not treat values of and as identical?

      • Mathieu Jobin

        I would like to know the answer to this exact question.

        thank you