EXPLAIN AND DESCRIBE:
mysql> explain select count(distinct seq) from Peptide;
+---------+------+---------------+------+---------+------+---------+-------+
| table   | type | possible_keys | key  | key_len | ref  | rows    | Extra |
+---------+------+---------------+------+---------+------+---------+-------+
| Peptide | ALL  | NULL          | NULL |    NULL | NULL | 1860342 |       |
+---------+------+---------------+------+---------+------+---------+-------+
1 row in set (0.02 sec)

mysql> desc Peptide;
+--------+---------------------------+------+-----+------------+----------------+
| Field  | Type                      | Null | Key | Default    | Extra          |
+--------+---------------------------+------+-----+------------+----------------+
| pepid  | int(10) unsigned zerofill |      | PRI | NULL       | auto_increment |
| proid  | int(10) unsigned zerofill |      | MUL | 0000000000 |                |
| seq    | tinytext                  |      | MUL |            |                |
| mh     | double                    |      |     | 0          |                |
| expect | double                    |      |     | 0          |                |
| start  | int(11)                   |      |     | 0          |                |
| end    | int(11)                   |      |     | 0          |                |
| charge | tinyint(1) unsigned       | YES  |     | NULL       |                |
| delta  | float                     |      |     | 0          |                |
| dida   | int(11)                   |      |     | 0          |                |
| didb   | int(11)                   |      |     | 0          |                |
| didc   | int(11)                   |      |     | 0          |                |
+--------+---------------------------+------+-----+------------+----------------+
12 rows in set (0.00 sec)