Question

I have some hierarchical data – each entry has an id and a (nullable) parent entry id. I want to retrieve all entries in the tree under a given entry. This is in a SQL Server 2005 database. I am querying it with LINQ to SQL in C# 3.5.

LINQ to SQL does not support Common Table Expressions directly. My choices are to assemble the data in code with several LINQ queries, or to make a view on the database that surfaces a CTE.

Which option (or another option) do you think will perform better when data volumes get large? Is SQL Server 2008’s HierarchyId type supported in Linq to SQL?

Answer 1

I would set up a view and an associated table-based function based on the CTE. My reasoning for this is that, while you could implement the logic on the application side, this would involve sending the intermediate data over the wire for computation in the application. Using the DBML designer, the view translates into a Table entity. You can then associate the function with the Table entity and invoke the method created on the DataContext to derive objects of the type defined by the view. Using the table-based function allows the query engine to take your parameters into account while constructing the result set rather than applying a condition on the result set defined by the view after the fact.

CREATE TABLE [dbo].[hierarchical_table](     [id] [int] IDENTITY(1,1) NOT NULL,     [parent_id] [int] NULL,     [data] [varchar](255) NOT NULL,  CONSTRAINT [PK_hierarchical_table] PRIMARY KEY CLUSTERED  (     [id] ASC )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY] ) ON [PRIMARY]  CREATE VIEW [dbo].[vw_recursive_view] AS WITH hierarchy_cte(id, parent_id, data, lvl) AS (SELECT     id, parent_id, data, 0 AS lvl       FROM         dbo.hierarchical_table       WHERE     (parent_id IS NULL)       UNION ALL       SELECT     t1.id, t1.parent_id, t1.data, h.lvl + 1 AS lvl       FROM         dbo.hierarchical_table AS t1 INNER JOIN                             hierarchy_cte AS h ON t1.parent_id = h.id) SELECT     id, parent_id, data, lvl FROM         hierarchy_cte AS result   CREATE FUNCTION [dbo].[fn_tree_for_parent]  (     @parent int ) RETURNS  @result TABLE  (     id int not null,     parent_id int,     data varchar(255) not null,     lvl int not null ) AS BEGIN     WITH hierarchy_cte(id, parent_id, data, lvl) AS    (SELECT     id, parent_id, data, 0 AS lvl         FROM         dbo.hierarchical_table         WHERE     (id = @parent OR (parent_id IS NULL AND @parent IS NULL))         UNION ALL         SELECT     t1.id, t1.parent_id, t1.data, h.lvl + 1 AS lvl         FROM         dbo.hierarchical_table AS t1 INNER JOIN             hierarchy_cte AS h ON t1.parent_id = h.id)     INSERT INTO @result     SELECT     id, parent_id, data, lvl     FROM         hierarchy_cte AS result RETURN  END  ALTER TABLE [dbo].[hierarchical_table]  WITH CHECK ADD  CONSTRAINT [FK_hierarchical_table_hierarchical_table] FOREIGN KEY([parent_id]) REFERENCES [dbo].[hierarchical_table] ([id])  ALTER TABLE [dbo].[hierarchical_table] CHECK CONSTRAINT [FK_hierarchical_table_hierarchical_table] 

To use it you would do something like — assuming some reasonable naming scheme:

using (DataContext dc = new HierarchicalDataContext()) {     HierarchicalTableEntity h = (from e in dc.HierarchicalTableEntities                                  select e).First();     var query = dc.FnTreeForParent( h.ID );     foreach (HierarchicalTableViewEntity entity in query) {         ...process the tree node...     } }