Question

我还在学习在SQL Server中阅读执行计划。我想知道是否有人可以告诉我以下哪个查询更快。

左连接：

SELECT  
    [InvoiceNum] = CONVERT(VARCHAR(10),H.Date, 101) + ' #'
    + RTRIM(SUBSTRING(RptText, CHARINDEX('No.', RptText) + 3, 25))
FROM    
    dbo.RptDisplayHeader H
LEFT JOIN 
    dbo.RptDisplayDetail d ON d.RptID =H.RptID
                           AND d.ID =H.ID
                           AND d.Date =H.Date
                           AND d.Seq = 2
WHERE   
   H.ID = @ID
    ANDH.RptID = @RptID
ORDER BY 
   H.Date DESC;

内联查询：

SELECT  
    [InvoiceNum] = CONVERT(VARCHAR(10),H.Date, 101)
    + (SELECT 
           ' #' + RTRIM(SUBSTRING(RptText, CHARINDEX('No.', RptText) + 3, 25))
       FROM    
           dbo.RptDisplayDetail d
       WHERE   
           d.ID = @ID
           AND d.RptID = @RptID
           AND d.Seq = 2
           AND d.Date =H.Date)
 FROM    
     dbo.RptDisplayHeaderH 
 WHERE   
    H.ID = @ID
     ANDH.RptID = @RptID
 ORDER BY 
    H.Date DESC;

执行计划：

内联统计：

  Table 'RptDisplayDetail'. Scan count 1723, logical reads 5444, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
  Table 'RptDisplayHeader'. Scan count 1, logical reads 58, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

左连接统计信息：

  Table 'RptDisplayDetail'. Scan count 1723, logical reads 5792, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
  Table 'RptDisplayHeader'. Scan count 1, logical reads 58, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

Answer 1

您需要比较最左边的select语句的Estimated Sub Tree成本（这是最后执行的操作）

成本最低的那个（理论上）是您最快的查询。

更可靠的衡量标准是您发布的IO统计信息。

根据您发布的统计信息，内联查询略有改善，因为RptDisplayDetail表上的逻辑读取较少。但是，就个人而言，我会取消这种边际收益，支持其他查询的可读性（除非您特别遇到性能问题）

有a great video over at BrentOzar.com在其中调整StackOverflow数据库上的示例查询

左连接vs行查询与执行计划

1 个答案: