为什么linqto实体要为我做一个子查询?

我使用.NET4和实体框架来构造一个简单的查询。以下是C代码:

return Context.Files.Where(f => f.FileHash != 40)
                    .OrderByDescending(f => f.Created)
                    .Take(5);

当我使用 ObjectQuery.ToTraceString()

SELECT TOP (5)
  [Project1].[ID] AS [ID],
  -- <snip> lots of columns
  [Project1].[FileHash] AS [FileHash]
FROM ( SELECT
         [Extent1].[ID] AS [ID],
         -- <snip> lots of columns
         [Extent1].[FileHash] AS [FileHash]
       FROM [dbo].[Files] AS [Extent1]
       WHERE (LEN([Extent1].[FileHash])) <> 40
)  AS [Project1]
ORDER BY [Project1].[Created] DESC

文件哈希 定义为 NVARCHAR(255) .

这对我来说似乎很奇怪,因为我认为不需要子查询。为什么EF要为我这样做,我能做些什么来避免我认为这样的查询会对性能造成影响?