MySQL查询不使用索引

Question

下面的查询：

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' 

使用说明后，很明显的查询使用表扫描：

有各种按键和pusher_idx同时包含device_id和flight_id 。为什么这个索引不被使用？

Answer 1

在这种情况下，MySQL优化改变了你的表的顺序（它可以做到这一点 - 他们都是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不受任何自己的谓词过滤（用新加盟为了加入其他表格，加入了其他表格，因此它们被过滤） - 它决定执行全面扫描。