mysql> explain select 1 from przedmiot order by id;
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table     | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | przedmiot | index | NULL          | PRIMARY | 4       | NULL | 6768 | Using index |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select 1 from przedmiot;
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table     | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | przedmiot | index | NULL          | PRIMARY | 4       | NULL | 6768 | Using index |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)