mysql - MySQL查询不使用索引

Question

以下查询：

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 Explain 后，很明显查询使用了表扫描：

有各种键，并pusher_idx包含device_id和flight_id。为什么不使用这个索引？

Answer 1

在这种情况下，mysql优化器改变了你的表的顺序（它可以做到——它们都是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，只要flights_followed没有被任何自己的谓词过滤（使用新的连接顺序，它是其他表连接到它，所以是它们被过滤） - 它决定执行完整扫描。