+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table    | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| zipcodes |          0 | PRIMARY  |            1 | id          | A         |       29356 |     NULL | NULL   |      | BTREE      |         |
| zipcodes |          1 | foo      |            1 | town        | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
| zipcodes |          1 | foo      |            2 | state       | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
| zipcodes |          1 | foo      |            3 | latitude    | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
| zipcodes |          1 | foo      |            4 | longitude   | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
| zipcodes |          1 | foo      |            5 | population  | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
| zipcodes |          1 | bar      |            1 | state       | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
| zipcodes |          1 | bar      |            2 | town        | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
8 rows in set (0.04 sec)

mysql> explain SELECT DISTINCT town, state, SUM(population)
    ->     AS pop, ACOS((SIN(RADIANS(AVG(latitude))) * SIN(RADIANS(42.878462333333)))
    ->     + (COS(RADIANS(AVG(latitude))) * COS(RADIANS(42.878462333333)) *
    ->     COS(RADIANS(AVG(longitude) - (-85.621038))))) * 3963.190275 AS dist FROM
    ->     zipcodes GROUP BY town, state ORDER BY dist LIMIT 10;
+----------+-------+---------------+------+---------+------+-------+---------------------------------+
| table    | type  | possible_keys | key  | key_len | ref  | rows  | Extra                           |
+----------+-------+---------------+------+---------+------+-------+---------------------------------+
| zipcodes | index | NULL          | foo  |      79 | NULL | 29356 | Using temporary; Using filesort |
+----------+-------+---------------+------+---------+------+-------+---------------------------------+
1 row in set (0.01 sec)