create user test_user@localhost;
create role test_role;
grant select on *.* to test_role;
grant test_role to test_user@localhost;
show grants;
Grants for test_user@localhost
GRANT `test_role` TO `test_user`@`localhost`
GRANT USAGE ON *.* TO `test_user`@`localhost`
set default role test_role;
select user, host, default_role from mysql.user;
ERROR 42000: SELECT command denied to user 'test_user'@'localhost' for table `mysql`.`user`
select user, host, default_role from mysql.user where user='test_user';
User	Host	default_role
test_user	localhost	test_role
show grants;
Grants for test_user@localhost
GRANT `test_role` TO `test_user`@`localhost`
GRANT USAGE ON *.* TO `test_user`@`localhost`
GRANT SELECT ON *.* TO `test_role`
SET DEFAULT ROLE `test_role` FOR `test_user`@`localhost`
select user, host, default_role from mysql.user where user='test_user';
User	Host	default_role
test_user	localhost	test_role
set default role NONE;
select user, host, default_role from mysql.user where user='test_user';
User	Host	default_role
test_user	localhost	
set default role invalid_role;
ERROR OP000: Invalid role specification `invalid_role`
select user, host, default_role from mysql.user where user='test_user';
User	Host	default_role
test_user	localhost	
select user, host, default_role from mysql.user;
ERROR 42000: SELECT command denied to user 'test_user'@'localhost' for table `mysql`.`user`
drop role test_role;
drop user test_user@localhost;