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)