慢速MySQL的查詢（使用IN）

Question

我有一個簡單的查詢：

SELECT Description, Temp1, Temp2, SensorData.DateAndTime 
FROM SensorData 
    INNER Join Sensors on Sensors.ChipId=SensorData.RockID 
WHERE SensorData.Id in (Select max(Id) LastRecord 
         From SensorData 
         Group by RockId) 
ORDER BY DhtTemp; 

它需要3秒。如果我刪除內部選擇查詢並將其替換爲查詢中的結果，則需要花費0.1秒。如果我單獨運行內部查詢，則需要.1秒。如果我刪除加入傳感器它運行在0.5秒（這將是好的。）任何建議？我有關於Id列的索引是INT。

Answer 1

ChipId in Sensors表用於連接條件和RockId在內部選擇查詢是group by列名，您是否試過對這些列進行索引？

在加法嘗試此查詢：

SELECT Description, Temp1, Temp2, myData.DateAndTime 
FROM 
    (SELECT *,max(id) as mymax FROM SensorData 
     Group by RockId 
     Order by id desc 
    ) as myData 
INNER Join Sensors on Sensors.ChipId= myData.RockID 
WHERE SensorData.Id = mydata.mymax 
ORDER BY DhtTemp; 

Answer 2

大多數where in (select ...)查詢可以（通過優化器經常自動，但不總是）被改寫爲連接;試試這個：

select Description, Temp1, Temp2, a.DateAndTime 
from SensorData a 
left join SensorData b on b.RockId = a.RockId 
    and b.ID > a.ID 
join Sensors on Sensors.ChipId = a.RockID 
where b.ID is null 
order by DhtTemp 

在英語中，這意味着「只返回沒有更高ID` SensorData行

確保在指數有RockId