什么是優化此SQL查詢的最佳方法

Question

我有以下SQL查詢，但是我注意到它給我的服務器帶來了壓力，因為每次運行它時，CPU使用率都會以20％的良好幅度躍升。

SELECT 
    c.name, c.billingaddress, c.billingcity, c.billingstate, c.billingzip,c.ifActive,
    (SELECT COUNT(l.id) FROM newLoads l WHERE l.idCompany = c.id AND l.smallStatus='1') as numberLoads,
    (SELECT (SUM(l.loadRate))/(SUM(l.esMiles)) FROM newLoads l WHERE l.idCompany = c.id AND l.loadRate != '0' AND l.esMiles != '0' AND l.smallStatus='1') as RPM
    FROM `companies` c WHERE ifContractor ='0' $cond
    ORDER BY numberLoads DESC

Answer 1

這可能會更有效：

SELECT  c.name, c.billingaddress, c.billingcity,
        c.billingstate, c.billingzip, c.ifActive,
        x.numberLoads, x.RPM
    FROM  
      ( SELECT  l.idCompany,
                COUNT(*) AS numberLoads,
                SUM(l.loadRate))/(SUM(l.esMiles) AS RPM
            FROM  newLoads l
            WHERE  l.smallStatus = '1' 
      ) AS x
    JOIN  companies AS c  ON c.id = x.idCompany
    WHERE  ifContractor = '0' $cond
    ORDER BY  x.numberLoads DESC; 

請提供SHOW CREATE TABLE和EXPLAIN SELECT ...

Answer 2

這是您的查詢：

SELECT c.name, c.billingaddress, c.billingcity, c.billingstate, c.billingzip, c.ifActive,
       (SELECT COUNT(l.id)
        FROM newLoads l
        WHERE l.idCompany = c.id AND l.smallStatus = '1'
       ) as numberLoads,
       (SELECT (SUM(l.loadRate))/(SUM(l.esMiles))
        FROM newLoads l
        WHERE l.idCompany = c.id AND l.loadRate <> '0' AND l.esMiles <> '0' AND l.smallStatus = '1'
       ) as RPM
FROM `companies` c
WHERE ifContractor = '0' $cond
ORDER BY numberLoads DESC;

我不知道$cond應該是什么。 它肯定不是有效的SQL語法，因此我將忽略它。

對於此查詢，您需要以下索引： companies(ifContractor, id)和newload(idCompany, smallstatus, loadrate, esmiles, id) 。

順便說一句，如果其值看起來像數字的列實際上是數字，則請刪除單引號。 類型轉換會混淆優化器。

Answer 3

也許20％並不那么糟糕？ （特別是如果只是短暫的爆發）從外觀上看，它可能需要處理大量數據才能獲得結果。

我試圖將newLoads表上的聚合合並到單個SELECT中，並最終得到（非常）類似於Rick James已經擁有的東西。 我的構造的另一個好處是，如果newLoads中沒有匹配的信息和/或當字段之一為零時，它與原始查詢更加一致。 （我認為，並沒有真正測試過）

SELECT c.name, c.billingaddress, c.billingcity, c.billingstate, c.billingzip, c.ifActive, agg.numberLoads, agg.RPM
FROM `companies` c
LEFT OUTER JOIN ( (SELECT l.idCompany,
                          numberLoads = COUNT(l.id),
                          RPM = (CASE WHEN SUM((CASE WHEN l.loadRate <> '0' AND l.esMiles <> '0' THEN 1 ELSE 0 END)) = 0 THEN NULL ELSE

                                    SUM((CASE WHEN l.loadRate <> '0' AND l.esMiles <> '0' THEN l.loadRate ELSE 0 END)) / SUM((CASE WHEN l.loadRate <> '0' AND l.esMiles <> '0' THEN l.esMiles ELSE 0 END)) 
                                        END)
                    FROM newLoads l
                   WHERE l.smallStatus = '1'
                   ) AS agg
             ON agg.idCompany = c.id 

WHERE c.ifContractor = '0' $cond
ORDER BY agg.numberLoads DESC;

無論如何，如果持續時間是一個問題，您可能要檢查是否在相關字段（如Gordon Linoff）上正確建議了（復合）索引，以及$cond可能包含的內容； 可能會知道正在執行哪種篩選以及它對查詢的整體性能有何影響。

PS：沒有太多的mysql實際經驗，我想知道l.esMiles <> '0'是否不比l.esMiles <> 0 “慢”，假設l.esMiles是一個數字字段（例如整數或十進制等。）