set global sql_mode="";
set local sql_mode="";
alter table mysql.user modify User char(16) binary not null default '';
alter table mysql.db modify User char(16) binary not null default '';
alter table mysql.tables_priv modify User char(16) binary not null default '';
alter table mysql.columns_priv modify User char(16) binary not null default '';
alter table mysql.procs_priv modify User char(16) binary not null default '';
alter table mysql.proc modify definer char(77) collate utf8_bin not null default '';
alter table mysql.event modify definer char(77) collate utf8_bin not null default '';
flush privileges;
select user();
Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
def					user()	253	77	14	N	1	39	8
user()
root@localhost
create user a17aaaaaaaaaaaaa0@localhost;
ERROR HY000: String 'a17aaaaaaaaaaaaa0' is too long for user name (should be no longer than 16)
grant usage on *.* to a17aaaaaaaaaaaaa0@lodalhost;
ERROR HY000: String 'a17aaaaaaaaaaaaa0' is too long for user name (should be no longer than 16)
drop user a17aaaaaaaaaaaaa0@lodalhost;
ERROR HY000: String 'a17aaaaaaaaaaaaa0' is too long for user name (should be no longer than 16)
alter table mysql.user modify User char(80) binary not null default '';
alter table mysql.db modify User char(80) binary not null default '';
alter table mysql.tables_priv modify User char(80) binary not null default '';
alter table mysql.columns_priv modify User char(80) binary not null default '';
alter table mysql.procs_priv modify User char(80) binary not null default '';
alter table mysql.proc modify definer char(141) collate utf8_bin not null default '';
alter table mysql.event modify definer char(141) collate utf8_bin not null default '';
flush privileges;
select user();
Catalog	Database	Table	Table_alias	Column	Column_alias	Type	Length	Max length	Is_null	Flags	Decimals	Charsetnr
def					user()	253	141	14	N	1	39	8
user()
root@localhost
set global sql_mode=default;
