避免在内部选择中进行全表扫描

Question

I have the following select in MySQL, which produces the right results but it takes unnecessarily long to execute:

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;

This query runs much slower than it should because it does a full table scan on tblLocations.

This is the part that really slows down the query:

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 

Here is the explain plan:

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  

It does a full table scan on tblLocations, even though I only need s small subset of DeviceID's in that table.

I just need to look at the DeviceID's that are returned from this part:

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

But unfortunately tblTrackedUsers.TrackedDeviceID is not visible in the inner select. So if I add

WHERE DeviceID = tblTrackedUsers.TrackedDeviceID

right above

GROUP BY DeviceID

It does not work.

How can I go about optimizing this query?

Structure of the tables involved with the relevant fields only:

tblGPSDevices:

---

ID | Email | Validated | Enabled

---

tblLocations:

---

ID | DeviceID | Lat | Lon | Radius | CreationTimeStamp

---

tblTrackedUsers:

---

ID | TrackerDeviceID | TrackedDeviceID | Validated

---

tblLocations.DeviceID, tblTrackedUsers.TrackerDeviceID and tblTrackedUsers.TrackedDeviceID are foreign keys pointing to tblGPSDevices.ID

What this query does:

The query should return all devices from tblGPSDevices that are being tracked by the user and their last location from tblLocations. The way to determine which devices are being tracked by a user is determined by tblTrackedUsers: select TrackedDeviceID from tblTrackedUsers where TrackerDeviceID = some_value

Answer 1

I did find the answer myself and I will post for future reference. This sped up the query from 2 sec. to 0.0179 sec. That is an enormous gain.

The key was to add one more inner select within:

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

in order to avoid a full table scan, since we are only interested in DeviceID's that are = to tblTrackedUsers.TrackedDeviceID. Now this select looks like this:

 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

Here is the full select now:

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;