Why did the following linq to sql query generate a subquery?

Question

I did the following query:

var list = from book in books
          where book.price > 50
          select book;

list = list.Take(50);

I would expect the above to generate something like:

SELECT top 50 id, title, price, author
FROM Books
WHERE price > 50

but it generates:

SELECT
[Limit1].[C1] as [C1]
[Limit1].[id] as [Id], 
[Limit1].[title] as [title], 
[Limit1].[price] as [price], 
[Limit1].[author]
FROM (SELECT TOP (50) 
             [Extent1].[id] as as [Id], 
             [Extent1].[title] as [title], 
             [Extent1].[price] as [price], 
             [Extent1].[author] as [author]
      FROM Books as [Extent1]
      WHERE [Extent1].[price] > 50
     ) AS [Limit1]

Why does the above linq query generate a subquery and where does the C1 come from?

Answer 1

Disclaimer: I've never used LINQ before...

My guess would be paging support? I guess you have some sort of Take(50, 50) method that gets 50 records, starting at record 50. Take a look at the SQL that query generates and you will probably find that it uses a similar sub query structure to allow it to return any 50 rows in a query in approximately the amount of time that it returns the first 50 rows.

In any case, the nested sub query doesn't add any performance overhead as it's automagically optimised away during compilation of the execution plan.