Why is Linq2SQL generating a nested query instead of using a JOIN? - sql-server-2008

Question

I'm trying to understand why Linq is generating the SQL that it is for the statement below:
var dlo = new DataLoadOptions();
dlo.LoadWith<TemplateNode>(x => x.TemplateElement);
db.LoadOptions = dlo;
var data = from node in db.TemplateNodes
where node.TemplateId == someValue
orderby node.Left
select node;
Which generates the following SQL:
SELECT [t2].[Id],
[t2].[ParentId],
[t2].[TemplateId],
[t2].[ElementId],
[t2].[Left] AS [Left],
[t2].[Right] AS [Right],
[t2].[Id2],
[t2].[Content]
FROM (SELECT ROW_NUMBER() OVER (ORDER BY [t0].[Left]) AS [ROW_NUMBER],
[t0].[Id],
[t0].[ParentId],
[t0].[TemplateId],
[t0].[ElementId],
[t0].[Left],
[t0].[Right],
[t1].[Id] AS [Id2],
[t1].[Content]
FROM [dbo].[TemplateNode] AS [t0]
INNER JOIN [dbo].[TemplateElement] AS [t1]
ON [t1].[Id] = [t0].[ElementId]
WHERE [t0].[TemplateId] = 16 /* #p0 */) AS [t2]
WHERE [t2].[ROW_NUMBER] > 1 /* #p1 */
ORDER BY [t2].[ROW_NUMBER]
There is a Foreign Key from TemplateNode.ElementId to TemplateElement.Id.
I would have expected the query to produce a JOIN, like so:
SELECT * FROM TemplateNode
INNER JOIN TemplateElement ON TemplateNode.ElementId = TemplateElement.Id
WHERE TemplateNode.TemplateId = #TemplateId
As per the suggestions in the answers to this question I have profiled both queries and the JOIN is 3 times faster than the nested query.
I'm using a .NET 4.0 Windows Forms app to test with SQL Server 2008 SP2 64bit developer edition.

Answer 1

The only reason that LINQ-SQL would generate the ROW_NUMBER query is due to the Skip Method. As bizare as the above SQL seems, I think within T-SQL there is no construct for simple paging like MySQL's Limit 10,25, so you get the above SQL when using Skip and Take.
I would assume that there is a Skip being used for paging purposes and LINQ-SQL is modifying the query. If you use an application like LINQ-Pad you can run different LINQ queries to see their generated SQL.

Answer 2

Your example of a join is not equivalent. You cannot get the ROW_NUMBER and subsequently select only rows WHERE ROW_NUMBER > 1 with a simple join. You would have to do a sub-select or similar to get this result.