Entity Framework Extensions Recursive Query

Question

There is a hierarchical query support in a popular EF extensions library: https://entityframework-extensions.net/to-self-hierarchy-list.

I'm curious how does it work under the hood? Is it getting handled by SQL server, meaning that the query is translated to CTE (common table expression)?

Answer 1

Disclaimer: I'm the owner of Entity Framework Extensions

You got it right, we indeed use a CTE in the generated SQL.

Here is how looks the SQL template:

WITH
    SelfHierarchyQueryOuter AS (@(sqlMaster)),
    SelfHierarchyQueryInner AS (@(sqlHierarchy)),
    SelfHierarchyQuery AS (SELECT A.*, 0 AS ZZZ_Recursion FROM (SELECT * FROM SelfHierarchyQueryOuter) AS A 
                                    UNION ALL
                                    SELECT B.*, ZZZ_Recursion + 1 AS ZZZ_Recursion FROM  (SELECT * FROM SelfHierarchyQueryInner) AS B
                                    INNER JOIN SelfHierarchyQuery AS C ON @(keyJoins) 
                                    WHERE ZZZ_Recursion < @(maxRecursion)
                                    )

@(selectFinal)
FROM    SelfHierarchyQuery

So a query like this one:

var list2 = context.EntitySimples.ToSelfHierarchyList(x => x.Parent, options => options.MaxRecursion = 5);

Will generate the following SQL:

WITH
    SelfHierarchyQueryOuter AS (SELECT TOP 100 PERCENT  
    [Extent1].[ID] AS [ID], 
    [Extent1].[ColumnInt1] AS [ColumnInt1], 
    [Extent1].[ColumnInt2] AS [ColumnInt2], 
    [Extent1].[Parent_ID] AS [Parent_ID]
    FROM [dbo].[EntitySimple] AS [Extent1]),
    SelfHierarchyQueryInner AS (SELECT TOP 100 PERCENT  
    [Extent1].[ID] AS [ID], 
    [Extent1].[ColumnInt1] AS [ColumnInt1], 
    [Extent1].[ColumnInt2] AS [ColumnInt2], 
    [Extent1].[Parent_ID] AS [Parent_ID]
    FROM [dbo].[EntitySimple] AS [Extent1]),
    SelfHierarchyQuery AS (SELECT A.*, 0 AS ZZZ_Recursion FROM (SELECT * FROM SelfHierarchyQueryOuter) AS A 
                                    UNION ALL
                                    SELECT B.*, ZZZ_Recursion + 1 AS ZZZ_Recursion FROM  (SELECT * FROM SelfHierarchyQueryInner) AS B
                                    INNER JOIN SelfHierarchyQuery AS C ON C.[Parent_ID] = B.[ID] 
                                    WHERE ZZZ_Recursion < 5
                                    )

SELECT *
FROM    SelfHierarchyQuery

Nothing "special" here, just using one great feature of the CTE to make it works.