MySQL usernames can be up to 16 characters long. This limit is hard-coded in the MySQL servers and clients, and trying to circumvent it by modifying the definitions of the tables in the mysql database does not work.
Note
You should never alter any of the tables in the mysql database in any manner whatsoever except by means of the procedure that is described in Section 4.4.9, “mysql_upgrade — Check Tables for MySQL Upgrade”. Attempting to redefine MySQL's system tables in any other fashion results in undefined (and unsupported!) behavior.
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