!和表值函数连接引发的性能问题分析

 表值函数

    SQL Server中提供了类似其他编程语言的函数，而函数的本质通常是一段代码的封装，并返回值。在SQL Server中，函数除了可以返回简单的数据类型之外（Int、Varchar等），还可以返回一个集合，也就是返回一个表。
    而根据是否直接返回集合或是定义后再返回集合，表值函数又分为内联用户定义表值函数和用户定义表值函数（下文统称为表值函数，省去“用户定义”四个字）。

内联表值函数
    内联表值函数和普通函数并无不同，唯一的区别是返回结果为集合（表），而不是简单数据类型，一个简单的内联表值函数如代码清单1所示（摘自VEVB）。

CREATE FUNCTION Sales.ufn_CustomerNamesInRegion( @Region nvarchar(50) )RETURNS tableASRETURN (SELECT DISTINCT s.Name AS Store, a.CityFROM Sales.Store AS sINNER JOIN Person.BusinessEntityAddress AS bea ON bea.BusinessEntityID = s.BusinessEntityID INNER JOIN Person.Address AS a ON a.AddressID = bea.AddressIDINNER JOIN Person.StateProvince AS sp ON sp.StateProvinceID = a.StateProvinceIDWHERE sp.Name = @Region);GO

代码清单1.一个简单的表值函数

用户定义表值函数
而用户定义表值函数，需要在函数开始时定义返回的表结构，然后可以写任何代码进行数据操作，插入到定义的表结构之后进行返回，一个稍微负责的用户定义表值函数示例如代码清单2所示（摘自VEVB）。

CREATE FUNCTION dbo.ufnGetContactInformation(@ContactID int) RETURNS @retContactInformation TABLE ( -- Columns returned by the function ContactID int PRIMARY KEY NOT NULL, FirstName nvarchar(50) NULL, LastName nvarchar(50) NULL, JobTitle nvarchar(50) NULL, ContactType nvarchar(50) NULL ) AS -- Returns the first name, last name, job title, and contact type for the specified contact. BEGIN DECLARE @FirstName nvarchar(50), @LastName nvarchar(50), @JobTitle nvarchar(50), @ContactType nvarchar(50); -- Get common contact information SELECT @ContactID = BusinessEntityID, @FirstName = FirstName, @LastName = LastName FROM Person.Person WHERE BusinessEntityID = @ContactID; -- Get contact job title SELECT @JobTitle = CASE -- Check for employee WHEN EXISTS(SELECT * FROM Person.Person AS p WHERE p.BusinessEntityID = @ContactID AND p.PersonType = 'EM') THEN (SELECT JobTitle FROM HumanResources.Employee AS e WHERE e.BusinessEntityID = @ContactID) -- Check for vendor WHEN EXISTS(SELECT * FROM Person.Person AS p WHERE p.BusinessEntityID = @ContactID AND p.PersonType = 'VC') THEN (SELECT ct.Name FROM Person.ContactType AS ct INNER JOIN Person.BusinessEntityContact AS bec ON bec.ContactTypeID = ct.ContactTypeID WHERE bec.PersonID = @ContactID)  -- Check for store WHEN EXISTS(SELECT * FROM Person.Person AS p WHERE p.BusinessEntityID = @ContactID AND p.PersonType = 'SC') THEN (SELECT ct.Name FROM Person.ContactType AS ct INNER JOIN Person.BusinessEntityContact AS bec ON bec.ContactTypeID = ct.ContactTypeID WHERE bec.PersonID = @ContactID) ELSE NULL END; -- Get contact type SET @ContactType = CASE -- Check for employee WHEN EXISTS(SELECT * FROM Person.Person AS p WHERE p.BusinessEntityID = @ContactID AND p.PersonType = 'EM') THEN 'Employee' -- Check for vendor WHEN EXISTS(SELECT * FROM Person.Person AS p WHERE p.BusinessEntityID = @ContactID AND p.PersonType = 'VC') THEN 'Vendor Contact' -- Check for store WHEN EXISTS(SELECT * FROM Person.Person AS p WHERE p.BusinessEntityID = @ContactID AND p.PersonType = 'SC') THEN 'Store Contact' -- Check for individual consumer WHEN EXISTS(SELECT * FROM Person.Person AS p WHERE p.BusinessEntityID = @ContactID AND p.PersonType = 'IN') THEN 'Consumer' -- Check for general contact WHEN EXISTS(SELECT * FROM Person.Person AS p WHERE p.BusinessEntityID = @ContactID AND p.PersonType = 'GC') THEN 'General Contact' END; -- Return the information to the caller IF @ContactID IS NOT NULL BEGIN INSERT @retContactInformation SELECT @ContactID, @FirstName, @LastName, @JobTitle, @ContactType; END; RETURN; END; GO