创建笛卡尔积时 CROSS APPLY 和 OUTER APPLY 有什么区别吗？

Question

在两个table之间创建笛卡尔积时，CROSS APPLY和OUTER APPLY之间有什么区别吗？

这可能看起来像一个愚蠢的问题，因为如果 table 之间没有表达关系，右手 table 不能不满足关系，但我很尊重我不知道的。

当我查看具有简单测试设置的执行计划时，它们是相同的[两个索引寻求馈入嵌套循环（内连接）]，但简单的测试设置可能具有欺骗性。

这是我的意思的一个例子 (SQL Fiddle)。设置：

CREATE TABLE dbo.First (
    Id      INT IDENTITY(1, 1) PRIMARY KEY,
    Name    NVARCHAR(100)
);
GO
DECLARE @n INT = 1;
WHILE @n < 10000
BEGIN
    INSERT INTO dbo.First (Name) VALUES ('First' + CONVERT(NVARCHAR(100), @n));
    SET @n = @n + 1;
END
GO
CREATE INDEX IX__First__Name ON dbo.First(Name);
GO
CREATE TABLE dbo.Second (
    Id      INT IDENTITY(1, 1) PRIMARY KEY,
    Name    NVARCHAR(100)
);
GO
DECLARE @n INT = 1;
WHILE @n < 10000
BEGIN
    INSERT INTO dbo.Second (Name) VALUES ('Second' + CONVERT(NVARCHAR(100), @n));
    SET @n = @n + 1;
END
GO
CREATE INDEX IX__Second__Name ON dbo.Second(Name);
GO

使用CROSS APPLY：

SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
CROSS APPLY Second
WHERE       First.Name IN ('First253', 'First3304')
AND         Second.Name IN ('Second6543', 'Second517');

使用OUTER APPLY:

SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
OUTER APPLY Second                                 -- <== Only change is here
WHERE       First.Name IN ('First253', 'First3304')
AND         Second.Name IN ('Second6543', 'Second517');

...两者都给我预期的四行。

加上各种变体，其中一个或两个 IN 子句 return 不匹配：

-- No match in First
SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
CROSS APPLY Second
WHERE       First.Name IN ('no match')
AND         Second.Name IN ('Second6543', 'Second517');

SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
OUTER APPLY Second
WHERE       First.Name IN ('no match')
AND         Second.Name IN ('Second6543', 'Second517');

-- No match in Second
SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
CROSS APPLY Second
WHERE       First.Name IN ('First253', 'First3304')
AND         Second.Name IN ('no match');

SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
OUTER APPLY Second
WHERE       First.Name IN ('First253', 'First3304')
AND         Second.Name IN ('no match');

-- No match in either
SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
CROSS APPLY Second
WHERE       First.Name IN ('no match')
AND         Second.Name IN ('no match');

SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
OUTER APPLY Second
WHERE       First.Name IN ('no match')
AND         Second.Name IN ('no match');

...所有这些都给我预期的零行。

Answer 1

当应用 table 或 table 值函数没有记录时，差异开始发挥作用：

SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
OUTER APPLY (SELECT * FROM Second WHERE Second.Id = -1) Second
WHERE       First.Name IN ('First253', 'First3304');

2 rows returned


SELECT      First.Id AS FirstId, Second.Id AS SecondId
FROM        First
CROSS APPLY (SELECT * FROM Second WHERE Second.Id = -1) Second
WHERE       First.Name IN ('First253', 'First3304');

0 rows returned

OP自己的话：

不是您这样做的方式，因为从概念上讲，您是在 APPLY 之后使用 WHERE 进行过滤（尽管计划显示引擎通过首先进行优化）；但是如果你先明确过滤然后 APPLY 像这样：

SELECT      First.Id AS FirstId, FilteredSecond.Id AS SecondId
FROM        First
CROSS APPLY (SELECT Id FROM Second WHERE Name IN ('xxx')) FilteredSecond 
WHERE       First.Name IN ('First253', 'First3304');

您会看到不同之处，因为您会得到包含 OUTER 的 NULL 行，但没有包含 CROSS.

的行

Answer 2

认为 CROSS APPLY 与 INNER JOIN 相关，OUTER APPLY 与 LEFT JOIN 相关。

CROSS / INNER 会将结果限制为来自两个来源的行，而
OUTER / LEFT 将 return 第一个 table 和 NULLs 的所有行。

不同之处在于 JOINs link 两个结果集基于一个关系条件，而 APPLY 被称为行方式，具有 当前行的值 .

您可以使用 APPLY 从行值中创建计算值，或者（这是主要目的）以行值作为参数调用 TVF。通常你可以看到 APPLY 与 XMLTypedVariable.nodes().

关于执行的一些思考 在简单的情况下（如上），引擎将决定走相同的路径。但是对于更复杂的场景，差异可能会很大。

创建笛卡尔积时 CROSS APPLY 和 OUTER APPLY 有什么区别吗？

Is there any difference between CROSS APPLY and OUTER APPLY when creating a cartesian product?

sql-server

tsql

cross-apply

outer-apply