ADO.NET Entity framework研究

    今天是我闲着没事，第一次正儿八经地研究Entity Framework，感觉没想象中那么好。根据我自己使用微软MSDN帮助中的School例子的分析来看，结果不尽人意。如下：

    对于关系型数据库，使用Ado.Net Entity framework，自带的数据查询方法，会导致大量数据库的操作。不适合用在大系统中，适合小网站、小企业应用的开发

    对一个表，本来只需要查询一个简单的列表，但Entity framework会将相关的所有表，都通过JOIN的方法，连接查询。而这个过程，据目前研究，对用户是透明的，比较难以修改。

    如微软MSDN帮助自带的School范例，本来只需要查询Department下的课程，但从Sql Server Profile中发现，第一次查询，会执行如下语句：

SELECT 

[Project2].[DepartmentID] AS [DepartmentID], 

[Project2].[Name] AS [Name], 

[Project2].[Budget] AS [Budget], 

[Project2].[StartDate] AS [StartDate], 

[Project2].[Administrator] AS [Administrator], 

[Project2].[C1] AS [C1], 

[Project2].[C2] AS [C2], 

[Project2].[CourseID] AS [CourseID], 

[Project2].[Title] AS [Title], 

[Project2].[Credits] AS [Credits], 

[Project2].[DepartmentID1] AS [DepartmentID1], 

[Project2].[CourseID1] AS [CourseID1], 

[Project2].[CourseID2] AS [CourseID2]

FROM ( SELECT 

 [Extent1].[DepartmentID] AS [DepartmentID], 

 [Extent1].[Name] AS [Name], 

 [Extent1].[Budget] AS [Budget], 

 [Extent1].[StartDate] AS [StartDate], 

 [Extent1].[Administrator] AS [Administrator], 

 1 AS [C1], 

 [Project1].[CourseID] AS [CourseID], 

 [Project1].[Title] AS [Title], 

 [Project1].[Credits] AS [Credits], 

 [Project1].[DepartmentID] AS [DepartmentID1], 

 [Project1].[CourseID1] AS [CourseID1], 

 [Project1].[CourseID2] AS [CourseID2], 

 [Project1].[C1] AS [C2]

 FROM  [dbo].[Department] AS [Extent1]

 LEFT OUTER JOIN  (SELECT 

  [Extent2].[CourseID] AS [CourseID], 

  [Extent2].[Title] AS [Title], 

  [Extent2].[Credits] AS [Credits], 

  [Extent2].[DepartmentID] AS [DepartmentID], 

  [Extent3].[CourseID] AS [CourseID1], 

  [Extent4].[CourseID] AS [CourseID2], 

  1 AS [C1]

  FROM   [dbo].[Course] AS [Extent2]

  LEFT OUTER JOIN [dbo].[OnlineCourse] AS [Extent3] ON [Extent2].[CourseID] = [Extent3].[CourseID]

  LEFT OUTER JOIN [dbo].[OnsiteCourse] AS [Extent4] ON [Extent2].[CourseID] = [Extent4].[CourseID] ) AS [Project1] ON [Extent1].[DepartmentID] = [Project1].[DepartmentID]

)  AS [Project2]

ORDER BY [Project2].[Name] ASC, [Project2].[DepartmentID] ASC, [Project2].[C2] ASC

这对大表来说，是个不可想像的操作。

当然，Entity framework使用了缓存，对于后续操作，未见其从数据库查询数据的动作。不过这只是一个Windows应用，至于web应用是什么情况，目前还不得而知。

由于Entity framework使用了数据缓存，但缓存大小、缓存生命期等的不可控性，目前还不好说Entity framework对系统的影响能有多大。

对于大型项目，特别是对性能要求特别高的项目，在没弄清楚Ado.Net Entity framework存在的各种风险之前，建议还是不要使用。对于小项目小网站，Ado.net EF倒是一个不错的选择，毕竟它的确带来了开发的便捷性。