Combined indexes may not work with OR. Try an index_merge in MySQL

Code
-- tested with MySQL 5.6

-- with combined index of timestamp and status
EXPLAIN SELECT id, name, status, date 
  FROM history
  WHERE timestamp = 1420934400 OR status = 3;

-- with separated indexes for timestamp and status
EXPLAIN SELECT id, name, status, date 
  FROM history
  WHERE timestamp = 1420934400 OR status = 3;
Result
idselect_typetablepartitionstypepossible_keyskeykey_lenrefrowsfilteredExtra
1SIMPLEhistorynullALLtest_idxnullnullnull140720.40Using where
0.001533031463623 ms (with caching enabled)

idselect_typetablepartitionstypepossible_keyskeykey_lenrefrowsfilteredExtra
1SIMPLEhistorynullindex_mergestatus_idx,timestamp_idx,test_idxtimestamp_idx,status_idx4,4null255100.00Using union(timestamp_idx,status_idx); Using where
0.0070168972015381 ms (with caching enabled)