MySQL查詢不使用索引

Question

下面的查詢：

EXPLAIN 
SELECT 
    fdb . * , 
    TIME_FORMAT(fdb.scheme, '%H:%i') AS scheme, 
    TIME_FORMAT(fdb.actual, '%H:%i') AS actual, 
    TIME_FORMAT(fdb.baggage, '%H:%i') AS baggage, 
    TIME_FORMAT(fdb.baggage_handled, '%H:%i') AS baggage_handled, 
    ff . * , TIME_FORMAT(ff.actual_saved, '%H:%i') AS actual_saved, 
    TIME_FORMAT(ff.baggage_saved, '%H:%i') AS baggage_saved, 
    TIME_FORMAT(ff.baggage_handled_saved, '%H:%i') AS baggage_handled_saved, 
    ap.device_id, 
    ap.device_token, 
    ap.device_language, 
    ap.app_edition, 
    ap.receive_status_notifications, 
    ap.receive_time_notifications, 
    ap.receive_luggage_notifications, 
    ap.receive_gate_notifications, 
    ap.receive_runway_notifications, 
    ap.receive_plane_notifications 
FROM flights_database fdb 
    JOIN flights_followed ff ON fdb.flight_id = ff.flight_id 
    JOIN apns_users ap ON ff.device_id = ap.device_id 
         AND ap.app_edition = '1' 

使用說明後，很明顯的查詢使用表掃描：

有各種按鍵和pusher_idx同時包含device_id和flight_id 。爲什麼這個索引不被使用？

Answer 1

在這種情況下，MySQL優化改變了你的表的順序（它可以做到這一點 - 他們都是INNER JOINS）：

SELECT 
    ... 
FROM flights_followed ff 
    JOIN flights_database fdb ON fdb.flight_id = ff.flight_id 
    JOIN apns_users ap ON ff.device_id = ap.device_id 
         AND ap.app_edition = '1' 

因此，一旦flights_followed不受任何自己的謂詞過濾（用新加盟爲了加入其他表格，加入了其他表格，因此它們被過濾） - 它決定執行全面掃描。