MySQL won't use an index on an int, if I provide a string Mysql randomly refuses to use indexes for even simple queries. 1 example  from slow-query log:


  # Time: 130904 12:19:59
   # User@Host: cron[cron] @  [10.71.41.38]
   # Query_time: 58.488760  Lock_time: 0.000029 Rows_sent: 0  Rows_examined: 13634899
  SET timestamp=1378311599;
  UPDATE dial_list set status='AB' where lead_id = '3335602076';


This is a MYISAM table with lead_id as primary key

| Field         | Type             | Null | Key | Default     |Extra                   |
| lead_id       | int(9) unsigned  | NO   | PRI | NULL        | auto_increment         |


The same query run on an identical server with similar data size uses the index. Also, not all such queries on this server turn up as table scans. What could be the problem?
From the comments dcaswell,


  Is the fact that you're comparing a string and int the issue


He suggests you check out 


http://stackoverflow.com/questions/16786063/mysql-comparison-of-integer-value-and-string-field-with-index