什么是优化此SQL查询的最佳方法

Question

我有以下SQL查询，但是我注意到它给我的服务器带来了压力，因为每次运行它时，CPU使用率都会以20％的良好幅度跃升。

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

这可能会更有效：

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; 

请提供SHOW CREATE TABLE和EXPLAIN SELECT ...

Answer 2

这是您的查询：

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应该是什么。 它肯定不是有效的SQL语法，因此我将忽略它。

对于此查询，您需要以下索引： companies(ifContractor, id)和newload(idCompany, smallstatus, loadrate, esmiles, id) 。

顺便说一句，如果其值看起来像数字的列实际上是数字，则请删除单引号。 类型转换会混淆优化器。

Answer 3

也许20％并不那么糟糕？ （特别是如果只是短暂的爆发）从外观上看，它可能需要处理大量数据才能获得结果。

我试图将newLoads表上的聚合合并到单个SELECT中，并最终得到（非常）类似于Rick James已经拥有的东西。 我的构造的另一个好处是，如果newLoads中没有匹配的信息和/或当字段之一为零时，它与原始查询更加一致。 （我认为，并没有真正测试过）

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;

无论如何，如果持续时间是一个问题，您可能要检查是否在相关字段（如Gordon Linoff）上正确建议了（复合）索引，以及$cond可能包含的内容； 可能会知道正在执行哪种筛选以及它对查询的整体性能有何影响。

PS：没有太多的mysql实际经验，我想知道l.esMiles <> '0'是否不比l.esMiles <> 0 “慢”，假设l.esMiles是一个数字字段（例如整数或十进制等。）