有沒有辦法加快這個查詢？

Question

我懷疑有一種方法可以使其更快，但它超出了我的MySQL限制。

我有一張表，它由從某些傳感器收集的數據組成，按活動爲基礎以1Hz的比率進行活動。表列是activityId，transducerId（數據來自哪個傳感器），傳感器正在報告的值以及時間戳。一個給定的活動可以有0 - 24個傳感器。

我需要一個新的表命名爲含該傳感器的數據的每個傳感器的列，和：等（取決於傳感器的數量給予或採取行）該數據的

一個第二看起來日期時間列。例如：

目前我得到這個表有一個很長的一系列查詢和連接。下面是我使用的查詢：

SELECT cd.calculatedValue AS `301`, q1.`302` , q2.`303` , q3.`304` , q4.`305` , q5.`306` , q6.`307` , q7.`308` , q8.`309` , q9.`310` , q10.`311` , q11.`312` , q12.`313` , q13.`314` , cd.`datetime` 
FROM 
data cd 
JOIN 
(SELECT `calculatedValue` AS `302`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 302) AS q1 
ON cd.`datetime` = q1.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `303`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 303) AS q2 
ON cd.`datetime` = q2.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `304`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 304) AS q3 
ON cd.`datetime` = q3.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `305`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 305) AS q4 
ON cd.`datetime` = q4.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `306`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 306) AS q5 
ON cd.`datetime` = q5.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `307`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 307) AS q6 
ON cd.`datetime` = q6.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `308`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 308) AS q7 
ON cd.`datetime` = q7.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `309`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 309) AS q8 
ON cd.`datetime` = q8.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `310`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 310) AS q9 
ON cd.`datetime` = q9.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `311`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 311) AS q10 
ON cd.`datetime` = q10.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `312`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 312) AS q11 
ON cd.`datetime` = q11.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `313`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 313) AS q12 
ON cd.`datetime` = q12.`datetime` 
JOIN 
(SELECT `calculatedValue` AS `314`, `datetime` FROM `data` WHERE `activityId` = 74 AND `transducerId` = 314) AS q13 
ON cd.`datetime` = q13.`datetime` 
WHERE cd.`activityId` = 74 AND cd.`transducerId` = 301 

這發生在短短几分鐘內數據的很長一段時間，切實會有數據的時間在表中，還有多達10多個傳感器。

有沒有更好的方法來做這個查詢？

非常感謝。

Answer 1

那些派生表將會在性能方面與你的午餐盒一起吃午餐。這些內聯視圖查詢會運行並物化爲臨時MyISAM表，然後外部查詢引用臨時MyISAM表（這些表未編制索引）來執行所有聯接操作。

作爲替代方案，考慮在表中使用一個鏡頭，獲得幾乎相同的結果。 （在你的查詢，如果日期時間行被用於任何換能器的「失蹤」，不返回任何一行。

考慮使用GROUP BY操作，這MySQL可能能夠使用合適的索引優化。

舉個例子，像這樣：

SELECT d.datetime 
    , MAX(IF(d.transducerId = 301,d.calculatedValue,NULL)) AS `301` 
    , MAX(IF(d.transducerId = 302,d.calculatedValue,NULL)) AS `302` 
    , MAX(IF(d.transducerId = 302,d.calculatedValue,NULL)) AS `302` 
    , MAX(IF(d.transducerId = 303,d.calculatedValue,NULL)) AS `303` 
    , MAX(IF(d.transducerId = 304,d.calculatedValue,NULL)) AS `304` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `305` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `306` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `307` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `308` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `309` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `310` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `311` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `312` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `313` 
    , MAX(IF(d.transducerId = 305,d.calculatedValue,NULL)) AS `314` 
    FROM `data` d 
WHERE d.activityId = 74 
GROUP BY d.datetime 

（您可以將d.datetime移動到SELECT列表的末尾，我通常有GROUP BY列第一。）

如果有並不是一個合適的指數，th查詢是否會像一條沉重的貨運列車一樣洶涌澎湃地吹着煙霧，掙扎着陡峭的等級。

此查詢的最合適的指數很可能將是

(activityID,datetime,transducerId,calculatedValue)

如果這是一個InnoDB表，並在集羣重點龍頭列(activityID,datetime)，這將是足夠的。

理想情況下，此查詢的EXPLAIN輸出在Extra列中顯示「使用where; using index」。我們絕對不想在EXPLAIN中看到的是「使用文件」操作，或任何派生表，我們可以幫助它。

---

該查詢與原始數據略有不同，如果特定日期時間某個特定傳感器的某一行「缺失」，則該查詢將返回該日期時間的一行，但「缺失」傳感器的空值將返回，原始查詢將忽略整行。

---

如果你確實想一起去JOIN操作，那麼相當於不利用內嵌的觀點會比原來更高效，雖然可能效率不高的GROUP BY查詢（在我的答案以上）。

SELECT cd301.datetime 
    , cd301.calculatedValue AS `301` 
    , cd302.calculatedValue AS `302` 
    , cd303.calculatedValue AS `303` 
    , cd304.calculatedValue AS `304` 
    , cd305.calculatedValue AS `305` 
    , cd306.calculatedValue AS `306` 
--  , cd307.calculatedValue AS `307` 
--  ... 
--  , cd314.calculatedValue AS `314` 
    FROM `data` cd301 
    JOIN `data` cd302 
    ON cd302.activityId = cd301.activityId 
    AND cd302.datetime  = cd301.datetime 
    AND cd302.transducerId = 302 
    JOIN `data` cd303 
    ON cd303.activityId = cd301.activityId 
    AND cd303.datetime  = cd301.datetime 
    AND cd303.transducerId = 303 
    JOIN `data` cd304 
    ON cd304.activityId = cd301.activityId 
    AND cd304.datetime  = cd301.datetime 
    AND cd304.transducerId = 304 
    JOIN `data` cd305 
    ON cd305.activityId = cd301.activityId 
    AND cd305.datetime  = cd301.datetime 
    AND cd305.transducerId = 305 
    JOIN `data` cd306 
    ON cd306.activityId = cd301.activityId 
    AND cd306.datetime  = cd301.datetime 
    AND cd306.transducerId = 306 
WHERE cd301.transducerId = 301 

很明顯，這需要擴展到307,308，... 314遵循相同的模式。

此外，此JOIN方法可能與GROUP BY等效，甚至更快，但與單行GROUP BY計劃相比，EXPLAIN將具有更多的行數。