Function on index

gistfile1.txt

mysql [localhost] {msandbox} (test) > CREATE TABLE cities (id int not null primary key auto_increment, name VARCHAR(50) NOT NULL, INDEX(name));
Query OK, 0 rows affected (0.05 sec)

mysql [localhost] {msandbox} (test) > INSERT INTO cities (name) VALUES ('Toronto'), ('Montreal'), ('Vancouver'), ('Calgary');
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql [localhost] {msandbox} (test) > update cities set name = AES_ENCRYPT(name, 'MYKEY');
Query OK, 4 rows affected (0.02 sec)
Rows matched: 4  Changed: 4  Warnings: 0

mysql [localhost] {msandbox} (test) > EXPLAIN SELECT * FROM cities WHERE name = AES_ENCRYPT('Toronto', 'MYKEY');
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table  | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | cities | NULL       | ref  | name          | name | 52      | const |    1 |   100.00 | Using index |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.02 sec)

mysql [localhost] {msandbox} (test) > EXPLAIN SELECT * FROM cities WHERE AES_DECRYPT(name, 'MYKEY')  = 'Toronto';
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
| id | select_type | table  | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | cities | NULL       | index | NULL          | name | 52      | NULL |    4 |   100.00 | Using where; Using index |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)