How to remove Table Spool(Eager Spool) from query plan of a function

Hi all.

Thanks for your time and expertise.

My question is about Table Spool operator

I have spent numerous hours reading all suggested topics, including topics when suggested during creation of this post.

The reason for Table Spool operator in query plan seems to be NOT an exact science but it's in the realm of magic.

I have a simple reproduction code to demonstrate an issue.

if you add Performance: Show Plan XML in profiler, then you will see something like this

I have appropriate indexes and queries are really simple.

I do not understand why this operator is there and is there any way to rid of it? I really would benefit from 44% gain.

Thank you.

use tempdb
go
BEGIN TRANSACTION
--DROP TABLE dbo.OrderItems
CREATE TABLE dbo.OrderItems
(
     itmID int NOT NULL
    ,itmIDInstance smallint NOT NULL
    ,itmIDParent int NULL
    ,itmIDParentInstance smallint NULL
    ,PRIMARY KEY (itmID, itmIDInstance)
)
INSERT dbo.OrderItems
VALUES
 (1, 1, NULL, NULL)
,(2, 1, 1, 1)

CREATE INDEX IX ON dbo.OrderItems (itmIDParent, itmIDParentInstance)
GO
--DROP FUNCTION dbo.fnAPP_getOrderItems
CREATE FUNCTION dbo.fnAPP_getOrderItems
(
     @itmID int
    ,@itmIDInstance smallint
)
RETURNS @oriopr TABLE 
(
     Lvl smallint NOT NULL
    ,itmID int NOT NULL
    ,itmIDInstance smallint NOT NULL
    ,PRIMARY KEY CLUSTERED
    (
         Lvl ASC
        ,itmID ASC
        ,itmIDInstance ASC
    )
)
AS
BEGIN;

    DECLARE @Lvl int = 0

    INSERT @oriopr
    SELECT
         @Lvl as Lvl
        ,ori.itmID
        ,ori.itmIDInstance
    FROM dbo.OrderItems ori
    WHERE ori.itmID = @itmID
    AND ori.itmIDInstance = @itmIDInstance
    WHILE @@ROWCOUNT > 0
    BEGIN
        SET @Lvl += 1

        INSERT @oriopr
        SELECT
             @Lvl as Lvl
            ,ori.itmID
            ,ori.itmIDInstance
        FROM dbo.OrderItems ori -- this level items
        JOIN @oriopr r -- parents
            ON r.itmID = ori.itmIDParent 
            AND r.itmIDInstance = ori.itmIDParentInstance
        WHERE r.Lvl = @Lvl - 1

    END

    RETURN

END
GO

SELECT * FROM dbo.fnAPP_getOrderItems (1, 1)


ROLLBACK

Vladimir Moldovanenko