SQL服務器 - OUTER APPLY與子查詢

Question

請考慮在SQL Server中的以下2個語句：

這一個是使用嵌套子查詢：

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 

而這種使用外部應用運算符：

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 

考慮到兩個Segments表可能有數百萬行，這兩個表中的哪一個可能執行得更好？

我的直覺是OUTER APPLY會表現更好。

一對夫婦的更多的問題：

1. 我幾乎敢肯定這一點，但還是想證實的是，在第一個方案中，CTE將有效地執行兩次（因爲其引用了兩次和熱膨脹係數像宏一樣內聯展開）。
2. 當在OUTER APPLY運算符中使用時，CTE是否會針對每行執行一次？當在第一個語句的嵌套查詢中使用時，它也會爲每一行執行？

Answer 1

首先，在CTE中擺脫Top 100 Percent。您在此處不使用TOP，如果您想要對結果進行排序，則應在整個聲明的末尾添加一個Order By。其次，爲了解決你關於性能的問題，如果被迫猜測，我的賭注只會在第二種形式，因爲它只有一個子查詢而不是兩個。第三，另一種形式，你可以嘗試將是：

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 

另一種可能性

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

也許我會用托馬斯的查詢的這種變化：

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