为什么群组通过使sql查询速度如此之快

以下两个查询给了我相同的结果，但使用group by运行速度更快。这是否表明优化查询将有利于在可能的情况下使用组？或者更可能是我遇到某种特殊情况？为什么群组通过使sql查询速度如此之快

更快的查询（不太直观我）：

SELECT  A.Advertiser 
FROM   (SELECT  TOP (100) PERCENT Advertiser, [Final Status] 
         FROM   dbo.Rehenas_View_2 
         GROUP BY [Final Status], Advertiser 
         HAVING  ([Final Status] IS NULL)) AS A INNER JOIN 
          (SELECT  TOP (100) PERCENT Advertiser, [Final Status] 
          FROM   dbo.Rehenas_View_2 AS Rehenas_View_2_1 
          GROUP BY [Final Status], Advertiser 
          HAVING  ([Final Status] = N'final')) AS B ON A.Advertiser = B.Advertiser 
GROUP BY A.Advertiser

慢查询（我正在努力简化以上时，我注意到了速度差）

SELECT  A.Advertiser 
FROM   (SELECT  TOP (100) PERCENT Advertiser, [Final Status] 
         FROM   dbo.Rehenas_View_2 
         WHERE  ([Final Status] IS NULL)) AS A INNER JOIN 
          (SELECT  TOP (100) PERCENT Advertiser, [Final Status] 
          FROM   dbo.Rehenas_View_2 AS Rehenas_View_2_1 
          WHERE  ([Final Status] = N'final')) AS B ON A.Advertiser = B.Advertiser 
GROUP BY A.Advertiser

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2010-12-07 Aaron Anodide

也许在两者上运行EXPLAIN PLAN都会给你一个提示。 – duffymo 2010-12-07 04:44:47

性能将取决于这是哪个数据库。当然看起来是MSSQL，但你应该用一个标签明确地声明它。 – 2010-12-07 04:50:05

两个查询似乎要复杂得多。难道他们不会给出与此相同的结果吗？

SELECT Advertiser 
FROM   dbo.Rehenas_View_2 
WHERE [Final Status] IS NULL 

UNION 

SELECT Advertiser 
FROM dbo.Rehenas_View_2 
WHERE [Final Status] = N'final'

这应该也快很多。

当然，无论何时从视图中进行选择，都无法查看查询本身，也必须查看视图sql。这与使用子查询几乎相同，除非它是物化视图。

来源

2010-12-07 04:49:27

如果在两个查询中消除最外面的组，您是否看到返回行数的差异？这可能可以解释它。

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2010-12-07 05:06:24 joelt

根据您的意见，您希望广告客户同时具有NULL和最终状态。

这应该呈现所需的结果。 DISTINCT通常意味着“我得到重复...不知道为什么”，通常它隐藏了一个迷你陷阱。在这种情况下，每个广告客户可能会有'最终'和NULL行的笛卡尔积。

SELECT DISTINCT A.Advertiser 
FROM dbo.Rehenas_View_2 AS A, 
     dbo.Rehenas_View_2 AS B 
WHERE A.[Final Status] IS NULL 
    AND B.[Final Status] = N'final' 
    AND A.Advertiser = B.Advertiser

INNER JOIN的2个骨料：

SELECT Advertiser FROM 
( SELECT Advertiser, COUNT(1) AS StatusCount 
    FROM dbo.Rehenas_View_2 WHERE [Final Status] IS NULL 
    GROUP BY Advertiser 
    HAVING StatusCount > 0) AS N, 
(
    SELECT Advertiser, COUNT(1) AS StatusCount 
    FROM dbo.Rehenas_View_2 WHERE [Final Status] = N'final' 
    GROUP BY Advertiser 
    HAVING StatusCount > 0) AS F 
WHERE N.Advertiser = F.Advertiser

另一个想法是用例和统计NULL和决赛用单GROUP BY/HAVING

SELECT Advertiser FROM 
( SELECT Advertiser, 
     SUM(CASE WHEN [Final Status] IS NULL THEN 1 ELSE 0 END) AS NullCount, 
     SUM(CASE WHEN [Final Status] = N'final' THEN 1 ELSE 0 END) AS FinalCount 
    FROM dbo.Rehenas_View_2 WHERE [Final Status] IS NULL 
    GROUP BY Advertiser 
    HAVING NullCount > 0 AND FinalCount > 0)

我没有测试此语法的环境。

来源

2010-12-07 09:43:21 kevpie

为什么群组通过使sql查询速度如此之快

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