Why SingleOrDefault result TOP(2) in SQL?

I am using EF4.0, and I wrote a query:

var query = context.Post.Where(p => p.Id == postId).SingleOrDefault();

I need only One post from this query. I thought SingleOrDefault() will generate "SELECT TOP(1) ...", but when I look into SQL Profiler, It was:

exec sp_executesql N'SELECT TOP (2) 
[Extent1].[Id] AS [Id], 
[Extent1].[Title] AS [Title], 
[Extent1].[Slug] AS [Slug], 
[Extent1].[PubDate] AS [PubDate], 
[Extent1].[PostContent] AS [PostContent], 
[Extent1].[Author] AS [Author], 
[Extent1].[CommentEnabled] AS [CommentEnabled], 
[Extent1].[AttachmentId] AS [AttachmentId], 
[Extent1].[IsPublished] AS [IsPublished], 
[Extent1].[Hits] AS [Hits], 
[Extent1].[CategoryId] AS [CategoryId]
FROM [dbo].[Post] AS [Extent1]
WHERE [Extent1].[Id] = @p__linq__0',N'@p__linq__0 uniqueidentifier',@p__linq__0='ECD9F3BE-3CA9-462E-AE79-2B28C8A16E32'

I wonder why EF result in SELECT TOP (2)? I only need one post.

It selects top 2 so that if there are actually 2 or more than 2 records in the database, an exception would be thrown. If it only selects top 1 there would be no way to error out.

By asking for the SingleOrDefault of a sequence, you are asking for this behaviour:

• if the sequence has exactly 0 elements, return the default for the sequence's element type
• if the sequence has exactly 1 element, return the element
• if the sequence has more than 1 element, throw

Doing a TOP (1) would empower the first two parts of this, but not the third. Only by doing a TOP (2) can we differentiate between exactly 1 record and more than 1 record.

If you don't want or need the third part of the above behviour, instead use FirstOrDefault.