I have an exact same myisam table on master and slave. The indexes are not being used on Master server.

mysql> explain select count(date_time) from ox_data_summary_ad_hourly where  date_time = '2012-03-08 00:00:00' \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: ox_data_summary_ad_hourly
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 32212292
        Extra: Using where
1 row in set (0.00 sec)

The explain plan on the slave:

mysql> explain select count(date_time) from ox_data_summary_ad_hourly where  date_time = '2012-03-08 00:00:00' \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: ox_data_summary_ad_hourly
         type: ref
possible_keys: ox_data_summary_ad_hourly_date_time
          key: ox_data_summary_ad_hourly_date_time
      key_len: 8
          ref: const
         rows: 156769
        Extra: Using index
1 row in set (0.00 sec)

The master is 5.0 while slave is on 5.5 version if that matters.

Does it mean the index file on master is corrupt?