SQL服务器 - OUTER APPLY与子查询

Question

请考虑在SQL Server中的以下2个语句：

这一个是使用嵌套子查询：

WITH cte AS 
(
    SELECT TOP 100 PERCENT * 
    FROM Segments 
    ORDER BY InvoiceDetailID, SegmentID 
) 
SELECT *, ReturnDate = 
       (SELECT TOP 1 cte.DepartureInfo 
        FROM cte 
        WHERE seg.InvoiceDetailID = cte.InvoiceDetailID 
         AND cte.SegmentID > seg.SegmentID), 
      DepartureCityCode = 
       (SELECT TOP 1 cte.DepartureCityCode 
        FROM cte 
        WHERE seg.InvoiceDetailID = cte.InvoiceDetailID 
         AND cte.SegmentID > seg.SegmentID) 
FROM Segments seg 

而这种使用外部应用运算符：

WITH cte AS 
(
    SELECT TOP 100 PERCENT * 
    FROM Segments 
    ORDER BY InvoiceDetailID, SegmentID 
) 
SELECT seg.*, t.DepartureInfo AS ReturnDate, t.DepartureCityCode 
FROM Segments seg OUTER APPLY (
       SELECT TOP 1 cte.DepartureInfo, cte.DepartureCityCode 
       FROM cte 
       WHERE seg.InvoiceDetailID = cte.InvoiceDetailID 
         AND cte.SegmentID > seg.SegmentID 
      ) t 

考虑到两个Segments表可能有数百万行，这两个表中的哪一个可能执行得更好？

我的直觉是OUTER APPLY会表现更好。

一对夫妇的更多的问题：

1. 我几乎敢肯定这一点，但还是想证实的是，在第一个方案中，CTE将有效地执行两次（因为其引用了两次和热膨胀系数像宏一样内联展开）。
2. 当在OUTER APPLY运算符中使用时，CTE是否会针对每行执行一次？当在第一个语句的嵌套查询中使用时，它也会为每一行执行？

Answer 1

首先，在CTE中摆脱Top 100 Percent。您在此处不使用TOP，如果您想要对结果进行排序，则应在整个声明的末尾添加一个Order By。其次，为了解决你关于性能的问题，如果被迫猜测，我的赌注只会在第二种形式，因为它只有一个子查询而不是两个。第三，另一种形式，你可以尝试将是：

With RankedSegments As 
    (
    Select S1.SegmentId, ... 
     , Row_Number() Over(Partition By S1.SegmentId Order By S2.SegmentId) As Num 
    From Segments As S1 
     Left Join Segments As S2 
      On S2.InvoiceDetailId = S1.InvoiceDetailId 
       And S2.SegmentId > S1.SegmentID 
    ) 
Select ... 
From RankedSegments 
Where Num = 1 

另一种可能性

With MinSegments As 
    (
    Select S1.SegmentId, Min(S2.SegmentId) As MinSegmentId 
    From Segments As S1 
     Join Segments As S2 
      On S2.InvoiceDetailId = S1.InvoiceDetailId 
       And S2.SegmentId > S1.SegmentID 
    Group By S1.SegmentId 
    ) 
Select ... 
From Segments As S1 
    Left Join (MinSegments As MS1 
     Join Segments As S2 
      On S2.SegmentId = MS1.MinSegmentId) 
     On MS1.SegmentId = S1.SegmentId 

Answer 2

也许我会用托马斯的查询的这种变化：

WITH cte AS 
(
SELECT *, Row_Number() Over(Partition By SegmentId Order By InvoiceDetailID, SegmentId) As Num 
FROM Segments) 
SELECT seg.*, t.DepartureInfo AS ReturnDate, t.DepartureCityCode 
FROM Segments seg LEFT JOIN cte t ON seg.InvoiceDetailID = t.InvoiceDetailID AND t.SegmentID > seg.SegmentID AND t.Num = 1