Covering index used despite missing column

I have the following query, using MariaDB 10 / InnoDB: SELECT id, sender_id, receiver_id, thread_id, date_created, content FROM user_message WHERE thread_id = 12345 AND placeholder = FALSE ORDER BY date_created DESC LIMIT 20 This query fetches messages according to the given conditions and sorts by date created. I have a covering index over (thread_id, date_created). When running EXPLAIN, the correct index is used and I get the output "Using where", although the query is using a column in the middle of the statement that is not in the index. I can use any value for "placeholder = x" and the result is the same. If I change the sorting to use another column, the EXPLAIN correctly indicates "Using where. Using filesort." I'm having a head-scratching moment. Could anyone please shed light on this? What I would expect to see is that an additional filesort would be needed as the covering index could not be completely used due to the additional column.