Question

我有一个关于由Entity Framework生成的sql和手写的问题。在我的项目中，我有一些实体（它们对于这个Q并不重要），举个简单的例子，当我使用这个代码时：

var query = context.Employees.Select(e => new {
                PersonalCode = e.PersonelCode,
                Fname = e.Person.Fname,
                Family = e.Person.Family,
                Email = e.Person.Emails
            });

生成的sql是这样的：

SELECT 
[Project1].[EmployeeID] AS [EmployeeID], 
[Project1].[EmployeeID1] AS [EmployeeID1], 
[Project1].[PersonID] AS [PersonID], 
[Project1].[EmployeeID2] AS [EmployeeID2], 
[Project1].[PersonID1] AS [PersonID1], 
[Project1].[PersonelCode] AS [PersonelCode], 
[Project1].[Fname] AS [Fname], 
[Project1].[Family] AS [Family], 
[Project1].[C1] AS [C1], 
[Project1].[EmailID] AS [EmailID], 
[Project1].[Mail] AS [Mail]
FROM ( SELECT 
    [Extent1].[EmployeeID] AS [EmployeeID], 
    [Extent1].[PersonelCode] AS [PersonelCode], 
    [Join1].[PersonID] AS [PersonID], 
    [Join1].[Fname] AS [Fname], 
    [Join1].[EmployeeID] AS [EmployeeID1], 
    [Join3].[PersonID] AS [PersonID1], 
    [Join3].[Family] AS [Family], 
    [Join3].[EmployeeID] AS [EmployeeID2], 
    [Join5].[EmailID1] AS [EmailID], 
    [Join5].[Mail] AS [Mail], 
    CASE WHEN ([Join5].[EmailID2] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
    FROM    [dbo].[Employees] AS [Extent1]
    LEFT OUTER JOIN  (SELECT [Extent2].[PersonID] AS [PersonID], [Extent2].[Fname] AS [Fname], [Extent3].[EmployeeID] AS [EmployeeID]
        FROM  [dbo].[Persons] AS [Extent2]
        LEFT OUTER JOIN [dbo].[Employees] AS [Extent3] ON [Extent2].[PersonID] = [Extent3].[EmployeeID] ) AS [Join1] ON [Extent1].[EmployeeID] = [Join1].[PersonID]
    LEFT OUTER JOIN  (SELECT [Extent4].[PersonID] AS [PersonID], [Extent4].[Family] AS [Family], [Extent5].[EmployeeID] AS [EmployeeID]
        FROM  [dbo].[Persons] AS [Extent4]
        LEFT OUTER JOIN [dbo].[Employees] AS [Extent5] ON [Extent4].[PersonID] = [Extent5].[EmployeeID] ) AS [Join3] ON [Extent1].[EmployeeID] = [Join3].[PersonID]
    LEFT OUTER JOIN  (SELECT [Extent6].[EmailID] AS [EmailID2], [Extent6].[PersonID] AS [PersonID], [Extent7].[EmailID] AS [EmailID1], [Extent7].[Mail] AS [Mail]
        FROM  [dbo].[EmailsForPersons] AS [Extent6]
        INNER JOIN [dbo].[Emails] AS [Extent7] ON [Extent6].[EmailID] = [Extent7].[EmailID] ) AS [Join5] ON [Join5].[PersonID] = [Extent1].[EmployeeID]
)  AS [Project1]
ORDER BY [Project1].[EmployeeID] ASC, [Project1].[EmployeeID1] ASC, [Project1].[PersonID] ASC, [Project1].[EmployeeID2] ASC, [Project1].[PersonID1] ASC, [Project1].[C1] ASC

但是通过这段代码：

SELECT     Employees.PersonelCode, Persons.Fname, Persons.Family, Emails.Mail
FROM         Employees 
                        LEFT OUTER JOIN     -- or: INNER JOIN
                        Persons ON Employees.EmployeeID = Persons.PersonID 
                                LEFT OUTER JOIN
                                EmailsForPersons ON Persons.PersonID = EmailsForPersons.PersonID 
                                            LEFT OUTER JOIN
                                            Emails ON EmailsForPersons.EmailID = Emails.EmailID

我会给出相同的结果！这些代码有什么区别？哪一个具有更高的性能和更高的速度？

Answer 1

您可以对两个查询进行分析和采样，看看哪个表现更好？

另见How to clean & optimise code generated by WCF OData service?

EF生成的SQL非常通用，需要在各种情况下工作。无论出于何种原因，它都非常冗长。它通常具有SELECT [Col1] FROM (SELECT [Col1] ...)嵌套结构，以及许多用于比较的CAST语句。

是否这样做是为了确保最大程度的兼容性以及某些人难以翻译的棘手查询的可能性最小，或者是否因为生成SQL的代码更加清晰和简单而完成，我们只能猜测。这是在实体框架团队中做出的设计决策。

坦率地说，除非使用查询分析器并排测试两个查询的性能，否则我根本不会担心这个问题。我希望两者之间的差异非常小。

如果生成的查询的性能更差，那么最简单的模式是将逻辑写入存储过程并让EF调用存储过程。这使得所有控制都远离EF，并将其交给您。

EF 4.1代码优先：EF生成的sql和自定义sql之间的区别

1 个答案: