Execution Plan Optimization when where clause is removed then added back

Question

I have a stored procedure that uses a table valued function which executes in 9 seconds. If I alter the table valued function and remove the where clause, the stored procedure executes in 3 seconds. If I add the where clause back, the query still executes in 3 seconds.

I took a look at the execution plans and it appears that after I remove the where clause, the execution plan includes parallelism and the scan count for 2 of my tables drops for 50000 and 65000 down to 5 and 3. After I add the where clause back, the optimized execution plan still runs unless I run DBCC FREEPROCCACHE.

Questions 1. Why would SQL Server start using the optimized execution plan for both queries only when I first remove the where clause?

1. Is there a way to force SQL Server to use this execution plan?

Also, this is a paramaterized all-in-one query that uses the (Parameter is null or Parameter) in the where clause, which I believe is bad for performance.

RETURNS TABLE 
AS
RETURN 
(
SELECT  TOP (@PageNumber * @PageSize)
                CASE
                    WHEN @SortOrder = 'Expensive' THEN ROW_NUMBER()     OVER (ORDER BY SellingPrice DESC)
                WHEN @SortOrder = 'Inexpensive' THEN ROW_NUMBER() OVER (ORDER BY SellingPrice ASC)                  
                WHEN @SortOrder = 'LowMiles' THEN ROW_NUMBER() OVER (ORDER BY Mileage ASC)
                WHEN @SortOrder = 'HighMiles' THEN ROW_NUMBER() OVER (ORDER BY Mileage DESC)
                WHEN @SortOrder = 'Closest' THEN ROW_NUMBER() OVER (ORDER BY P1.Distance ASC)       
                WHEN @SortOrder = 'Newest' THEN ROW_NUMBER() OVER (ORDER BY [Year] DESC)    
                WHEN @SortOrder = 'Oldest' THEN ROW_NUMBER() OVER (ORDER BY [Year] ASC)                     
                ELSE ROW_NUMBER() OVER (ORDER BY InventoryID ASC)
            END as rn,
            P1.InventoryID,
            P1.SellingPrice,
            P1.Distance,
            P1.Mileage,
            Count(*) OVER () RESULT_COUNT,
            dimCarStatus.[year]
    FROM    (SELECT InventoryID, SellingPrice, Zip.Distance, Mileage, ColorKey, CarStatusKey, CarKey FROM facInventory
                JOIN @ZipCodes Zip
                ON   Zip.DealerKey = facInventory.DealerKey) as P1
    JOIN    dimColor
            ON dimColor.ColorKey = P1.ColorKey
    JOIN    dimCarStatus
            ON dimCarStatus.CarStatusKey = P1.CarStatusKey  
    JOIN    dimCar
            ON dimCar.CarKey = P1.CarKey                        
    WHERE
            (@ExteriorColor is NULL OR dimColor.ExteriorColor like @ExteriorColor) AND
            (@InteriorColor is NULL OR dimColor.InteriorColor like @InteriorColor) AND
            (@Condition is NULL OR dimCarStatus.Condition like @Condition) AND
            (@Year is NULL OR dimCarStatus.[Year] like @Year) AND
            (@Certified is NULL OR dimCarStatus.Certified like @Certified) AND
            (@Make is NULL OR dimCar.Make like @Make) AND
            (@ModelCategory is NULL OR dimCar.ModelCategory like @ModelCategory) AND    
            (@Model is NULL OR dimCar.Model like @Model) AND
            (@Trim is NULL OR dimCar.Trim like @Trim) AND
            (@BodyType is NULL OR dimCar.BodyType like @BodyType) AND
            (@VehicleTypeCode is NULL OR dimCar.VehicleTypeCode like @VehicleTypeCode) AND
            (@MinPrice is NULL OR P1.SellingPrice >= @MinPrice) AND
            (@MaxPrice is NULL OR P1.SellingPrice < @MaxPrice) AND
            (@Mileage is NULL OR P1.Mileage < @Mileage)
    ORDER   BY
            CASE
                WHEN @SortOrder = 'Expensive' THEN -SellingPrice
                WHEN @SortOrder = 'Inexpensive' THEN SellingPrice 
                WHEN @SortOrder = 'LowMiles' THEN Mileage
                WHEN @SortOrder = 'HighMiles' THEN -Mileage
                WHEN @SortOrder = 'Closest' THEN P1.Distance        
                WHEN @SortOrder = 'Newest' THEN -[YEAR]
                WHEN @SortOrder = 'Oldest' THEN [YEAR]                  
                ELSE InventoryID 
            END
)

Answer 1

Question 1: SQL server still continues using execution plan valid for statement without where clause. Basically SQL Server thinks that where clause is not there.

Question 2: Use OPTION (USE PLAN N'') article: http://technet.microsoft.com/en-us/library/cc917694.aspx

Personal recommendation:

1. Change the query so you have at least one mandatory filter and index it. or
2. Change it to be dynamic, and apply filters dynamically. Article: http://msdn.microsoft.com/en-us/library/ms188001.aspx

Hope this helps.