select子句中的子查询vs应用运算符性能

Question

我在select子句和外部apply中测试相关子查询的性能。为什么select子句中的相关子查询的执行效果优于OUTER APPLY

- 选择---

中的相关子查询

SELECT
    (SELECT TOP 1 
         L.LineNum
     FROM RDR1 L
     WHERE T1.Docentry = L.DocEntry
     ORDER BY L.Linenum DESC) as LineNum,
   T1.Docentry, T1.Docdate, T1.CardCode, *
FROM 
   ORDR T1

--36 sec--

- 外部申请 -

SELECT
   L.LineNum,T1.Docentry,T1.Docdate,T1.CardCode,*
FROM 
   ORDR T1
OUTER APPLY
   (SELECT TOP 1 
        T2.LineNum
    FROM RDR1 T2
    WHERE T1.Docentry = T2.DocEntry
    ORDER BY T2.Linenum DESC) as L
--42sec--

Answer 1

您是否查看了查询计划

Select max(T2.LineNum), T1.Docentry, T1.Docdate, T1.CardCode
  FROM ORDR T1
  join RDR1 t2 
    on T1.Docentry = T2.DocEntry 
 group by T1.Docentry, T1.Docdate, T1.CardCode