mysql> show fields from mysql.db;
ERROR 126 (HY000): Incorrect key file for table '/tmp/#sql_75ff_0.MYI'; try to repair it
mysql> repair table mysql.db;
+----------+--------+----------+----------+
| Table    | Op     | Msg_type | Msg_text |
+----------+--------+----------+----------+
| mysql.db | repair | status   | OK       | 
+----------+--------+----------+----------+
1 row in set (0.00 sec)

mysql> show fields from mysql.db;
ERROR 126 (HY000): Incorrect key file for table '/tmp/#sql_75ff_0.MYI'; try to repair it
mysql> select count(*) from mysql.db;
+----------+
| count(*) |
+----------+
|       17 | 
+----------+
1 row in set (0.00 sec)