gistfile1.txt

mysql56> CREATE TABLE t1 (num int, val varchar(32), key(num));
Query OK, 0 rows affected (0.03 sec)

mysql56>
mysql56> INSERT INTO t1 VALUES (1, 'one');
Query OK, 1 row affected (0.01 sec)

mysql56>
mysql56> EXPLAIN SELECT * FROM t1 WHERE num = 1;
+----+-------------+-------+------+---------------+------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref   | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------+
|  1 | SIMPLE      | t1    | ref  | num           | num  | 5       | const |    1 | NULL  |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql56> EXPLAIN SELECT * FROM t1 WHERE num = 1 ORDER BY val;
+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref   | rows | Extra                       |
+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------------------+
|  1 | SIMPLE      | t1    | ref  | num           | num  | 5       | const |    1 | Using where; Using filesort |
+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------------------+
1 row in set (0.00 sec)

mysql56>
mysql56> ALTER TABLE t1 ADD UNIQUE KEY (num);
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql56>
mysql56> EXPLAIN SELECT * FROM t1 WHERE num = 1 ORDER BY val;
+----+-------------+-------+-------+---------------+-------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key   | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+-------+---------+-------+------+-------+
|  1 | SIMPLE      | t1    | const | num_2,num     | num_2 | 5       | const |    1 | NULL  |
+----+-------------+-------+-------+---------------+-------+---------+-------+------+-------+
1 row in set (0.00 sec)