The following occurred at 10:21 this morning. Identifying details have been changed to protect the innocent.

> SELECT …;
10 rows in set (22.95 sec)

> DESCRIBE …;
+------------+---------+------+-----+---------+-------+
| Field      | Type    | Null | Key | Default | Extra |
+------------+---------+------+-----+---------+-------+
| ...        | int(11) | YES  |     | NULL    |       |
| ...        | int(11) | YES  |     | NULL    |       |
+------------+---------+------+-----+---------+-------+
2 rows in set (0.01 sec)

> ALTER TABLE … ADD PRIMARY KEY (…);
Query OK, 784 rows affected (0.43 sec)
Records: 784 Duplicates: 0 Warnings: 0

> SELECT …;
10 rows in set (0.09 sec)