避免内部选择中的全表扫描

Question

我在 MySQL 中有以下选择，它产生了正确的结果，但执行起来花费了不必要的时间：

SELECT tblGPSDevices.Email, tblLoc.Lat, tblLoc.Lon, tblLoc.Radius, tblLoc.CreationTimeStamp, tblTrackedUsers.ID, tblTrackedUsers.TrackerDeviceID, tblTrackedUsers.TrackedDeviceID
    FROM tblTrackedUsers
    INNER JOIN tblGPSDevices ON tblTrackedUsers.TrackedDeviceID = tblGPSDevices.ID
    LEFT OUTER JOIN (
            SELECT A.DeviceID, A.Lat, A.Lon, A.Radius, A.CreationTimeStamp, A.ID 
            FROM tblLocations A 
            INNER JOIN (
                    SELECT DeviceID, MAX(CreationTimeStamp) AS CreationTimeStamp, MAX(ID) AS ID
                    FROM tblLocations
                    GROUP BY DeviceID
            ) AS B ON A.DeviceID = B.DeviceID
                AND A.CreationTimeStamp = B.CreationTimeStamp
                AND A.ID = B.ID                         
    ) AS tblLoc ON tblLoc.DeviceID = tblGPSDevices.ID
    WHERE tblGPSDevices.Validated = 0x01
    AND tblGPSDevices.Enabled = 0x01
    AND tblTrackedUsers.Validated = 0x01
    AND tblTrackedUsers.TrackerDeviceID = 1
    ORDER BY tblTrackedUsers.ID;

此查询的运行速度比应有的要慢得多，因为它对 tblLocations 进行全表扫描。

这是真正减慢查询速度的部分：

SELECT A.DeviceID, A.Lat, A.Lon, A.Radius, A.CreationTimeStamp, A.ID 
        FROM tblLocations A 
        INNER JOIN (
            SELECT DeviceID, MAX(CreationTimeStamp) AS CreationTimeStamp, MAX(ID) AS ID
            FROM tblLocations
            GROUP BY DeviceID
        ) AS B ON A.DeviceID = B.DeviceID
            AND A.CreationTimeStamp = B.CreationTimeStamp
            AND A.ID = B.ID 

这里是解释计划：

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   PRIMARY     tblTrackedUsers     ref     TrackerDeviceID,TrackedDeviceID     TrackerDeviceID     9   const   14  Using where; Using temporary; Using filesort
1   PRIMARY     tblGPSDevices   eq_ref  PRIMARY     PRIMARY     8   tblTrackedUsers.TrackedDeviceID     1   Using where
1   PRIMARY     <derived2>  ALL     NULL    NULL    NULL    NULL    2073    
2   DERIVED     <derived3>  ALL     NULL    NULL    NULL    NULL    2073    
2   DERIVED     A   eq_ref  PRIMARY,DeviceID,CreationTimeStampIndex     PRIMARY     8   B.ID    1   Using where
3   DERIVED     tblLocations    index   NULL    DeviceID    8   NULL    174058  

它对 tblLocations 进行全表扫描，尽管我只需要该表中的一小部分 DeviceID。

我只需要查看从这部分返回的 DeviceID：

INNER JOIN tblGPSDevices ON tblTrackedUsers.TrackedDeviceID = tblGPSDevices.ID
WHERE tblTrackedUsers.TrackerDeviceID = 1

但不幸的是，tblTrackedUsers.TrackedDeviceID 在内部选择中不可见。所以如果我添加

WHERE DeviceID = tblTrackedUsers.TrackedDeviceID

正上方

GROUP BY DeviceID

它不起作用。

我该如何优化这个查询？

仅涉及相关字段的表结构：

tblGPSDevices：

---

ID |电子邮件 |已验证 |已启用

---

tblLocations：

---

ID |设备ID |纬度 |朗 |半径 |创建时间戳

---

tblTrackedUsers：

---

ID |跟踪设备ID |跟踪设备ID |已验证

---

tblLocations.DeviceID、tblTrackedUsers.TrackerDeviceID 和 tblTrackedUsers.TrackedDeviceID 是指向 tblGPSDevices.ID 的外键

此查询的作用：

查询应返回用户正在跟踪的 tblGPSDevices 中的所有设备以及 tblLocations 中的最后位置。确定用户正在跟踪哪些设备的方式由 tblTrackedUsers 确定：select TrackedDeviceID from tblTrackedUsers where TrackerDeviceID = some_value

Answer 1

我自己确实找到了答案，我将发布以供将来参考。这从 2 秒加快了查询速度。到 0.0179 秒。这是一个巨大的收获。

关键是在内部添加一个内部选择：

SELECT DeviceID, MAX(CreationTimeStamp) AS CreationTimeStamp, MAX(ID) AS ID
FROM tblLocations
GROUP BY DeviceID

为了避免全表扫描，因为我们只对 = 到 tblTrackedUsers.TrackedDeviceID 的 DeviceID 感兴趣。现在这个选择看起来像这样：

 SELECT C.DeviceID, MAX(C.CreationTimeStamp) AS CreationTimeStamp, MAX(C.ID) AS ID
 FROM tblLocations C
         INNER JOIN (
              SELECT ID, TrackerDeviceID, TrackedDeviceID,  TrackedName, AccessCode, Validated FROM tblAllowedUsers WHERE TrackerDeviceID = 1 AND Validated=0x01
          ) AS D ON D.TrackedDeviceID = C.DeviceID
  GROUP BY DeviceID

现在是完整的选择：

SELECT tblGPSDevices.Email, tblLoc.Lat, tblLoc.Lon, tblLoc.Radius, tblLoc.CreationTimeStamp, tblTrackedUsers.ID, tblTrackedUsers.TrackerDeviceID, tblTrackedUsers.TrackedDeviceID
    FROM tblTrackedUsers
    INNER JOIN tblGPSDevices ON tblTrackedUsers.TrackedDeviceID = tblGPSDevices.ID
    LEFT OUTER JOIN (
            SELECT A.DeviceID, A.Lat, A.Lon, A.Radius, A.CreationTimeStamp, A.ID 
            FROM tblLocations A 
            INNER JOIN (
                SELECT C.DeviceID, MAX(C.CreationTimeStamp) AS CreationTimeStamp, MAX(C.ID) AS ID
            FROM tblLocations C
                INNER JOIN (
                    SELECT ID, TrackerDeviceID, TrackedDeviceID,  TrackedName, AccessCode, Validated FROM tblAllowedUsers WHERE TrackerDeviceID = 1 AND Validated=0x01
                ) AS D ON D.TrackedDeviceID = C.DeviceID
            GROUP BY DeviceID
            ) AS B ON A.DeviceID = B.DeviceID
                AND A.CreationTimeStamp = B.CreationTimeStamp
                AND A.ID = B.ID                         
    ) AS tblLoc ON tblLoc.DeviceID = tblGPSDevices.ID
    WHERE tblGPSDevices.Validated = 0x01
    AND tblGPSDevices.Enabled = 0x01
    AND tblTrackedUsers.Validated = 0x01
    AND tblTrackedUsers.TrackerDeviceID = 1
    ORDER BY tblTrackedUsers.ID;