 <p>MySQL can only use one index in a case like that. It will choose either the platform_id index or the master_id index, depending on which one has the highest cardinality (number of distinct values in the index.) You can see the cardinality of each index with <code>SHOW INDEXES</code>. It is possible that the values MySQL has stored for cardinality aren't correct, in which case you can force it to recalculate them with <code>ANALYZE TABLE main</code>, but probably the results you're seeing are just because the distribution of unique values in those two indexes are such that it's not picking the most efficient one. Using a composite index as you've done here is probably the best strategy.</p>