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在Entity Framework中，如果實體A關聯了實體B，你想在加載實體A的同時加載實體B。通常做法是在LINQ查詢中使用Include()。但是，如果你在查詢條件中用到了實體B，EF會自動加載實體B，這時Include不僅是多余的，而且還會增加額外的LEFT OUTER JOIN查詢，影響性能。?

請看我們在博問開發中遭遇這個問題時的一段代碼：

//For q.cnblogs.com public class QuestionService {private IRepository<QuestionItem> _questionRepository;public QuestionService(IUnitOfWork unitOfWork): base(unitOfWork){_questionRepository = new Repository<QuestionItem>(unitOfWork);}public List<QuestionItem> GetUnsolvedQuestions(int pageIndex, int pageSize){return _questionRepository.Entities.Include(q => q.User).Where(q => q.IsActive && q.User.IsActive).Skip((pageIndex - 1) * pageSize).Take(pageSize).ToList();} }public class QuestionItem {public int Id { get;set; }public string Title { get; set; }public bool IsActive { get; set; }public int UserId { get; set; }public User User { get; set; } }public class User {public int UserId { get; set; }public bool IsActive {get;set;} }

在上面的代碼中，我們想在GetActiveQuestions()返回List<QuestionItem>時，QuestionItem中要包含User信息，所以在LINQ查詢使用了.Include(q => q.User)。

（特別要注意的是：這里把q.User.IsActive作為查詢條件之一）

然后我們用SQL Server Profiler發現，Entity Framework生成了如下的SQL語句：

SELECT TOP (25) [Filter1].[Id] AS [Id], [Filter1].[Title] AS [Title], [Filter1].[UserId] AS [UserId], [Filter1].[UserId1] AS [UserId1], [Filter1].[IsActive1] AS [IsActive], [Filter1].[UserId2] AS [UserId1], [Filter1].[IsActive2] AS [IsActive1] FROM ( SELECT [Extent1].[Id] AS [Id], [Extent1].[Title] AS [Title], [Extent1].[UserId] AS [UserId1],[Extent1].[IsActive] AS [IsActive1], [Extent3].[UserID] AS [UserID2], [Extent3].[IsActive] AS [IsActive2], row_number() OVER (ORDER BY [Extent1].[QID] DESC) AS [row_number]FROM [dbo].[question_Item] AS [Extent1]INNER JOIN [dbo].[Users] AS [Extent2] ON [Extent1].[UserID] = [Extent2].[UserID]LEFT OUTER JOIN [dbo].[Users] AS [Extent3] ON [Extent1].[UserID] = [Extent3].[UserID]WHERE ([Extent1].[IsActive] = 1) AND ([Extent2].[IsActive] = 1) ) AS [Filter1] WHERE [Filter1].[row_number] > 0 ORDER BY [Filter1].[Id] DESC

[dbo].[Users]表對應的就是User實體類，上面的SQL中[dbo].[Users]出現了兩次JOIN，由于[Users]表數據量比較大，兩次JOIN影響了執行計劃，查詢耗時增加。這顯然是要避免的。

在與這個問題一陣激戰之后，我們終于找到解決方法 —— 去掉Include，就這么簡單！

從這個地方看，Entity Framework還是挺聰明的，但是由于不知道它的這個聰明之處，反而帶來了問題。

所以，代碼如人，要和她相處好，就要了解她的一切！

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原文：http://www.cnblogs.com/dudu/archive/2012/04/13/entity_framework_include_where.html

轉載于:https://www.cnblogs.com/lenther2002/p/4614758.html

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