Filtered index is not used when variable in WHERE condition Why MS SQL Server refuse using supporting filtered index in this scenario?

-- demo data
CREATE TABLE #Test (
    ID INT IDENTITY(1,1) NOT NULL CONSTRAINT PK_Test_ID PRIMARY KEY
    ,Col1 NVARCHAR(36) NOT NULL DEFAULT NEWID()
    ,Col2 NVARCHAR(20) NOT NULL DEFAULT N''  -- !!
    );

WITH
    L0   AS(SELECT 1 AS C UNION ALL SELECT 1 AS O), -- 2 rows
    L1   AS(SELECT 1 AS C FROM L0 AS A CROSS JOIN L0 AS B), -- 4 rows
    L2   AS(SELECT 1 AS C FROM L1 AS A CROSS JOIN L1 AS B), -- 16 rows
    L3   AS(SELECT 1 AS C FROM L2 AS A CROSS JOIN L2 AS B), -- 256 rows
    L4   AS(SELECT 1 AS C FROM L3 AS A CROSS JOIN L3 AS B), -- 65,536 rows
    L5   AS(SELECT 1 AS C FROM L4 AS A CROSS JOIN L4 AS B), -- 4,294,967,296 rows
    Nums AS(SELECT ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) AS N FROM L5)
INSERT INTO #Test(Col2)
SELECT TOP 100000 N''
FROM Nums;

INSERT INTO #Test(Col2)
VALUES(N'ABC');

-- FILTERED index to support filter predicate of a query
CREATE NONCLUSTERED INDEX IX_Test_Col2_filtered ON #Test (Col2 ASC) WHERE Col2 &lt;&gt; N'';

-- just checking statistics
DBCC SHOW_STATISTICS('#Test', 'IX_Test_Col2_filtered')

-- condition on variable = index scan :-(
DECLARE @Filter NVARCHAR(20) = N'ABC'

SELECT Col1
FROM #Test
WHERE Col2 = @Filter
    AND Col2 &lt;&gt; N'';




Everything goes as expected when using literals.

-- condition on literal value - index seek + key lookup :-)
SELECT Col1
FROM #Test
WHERE Col2 = N'ABC';
Filtered indexes can't use variables/parameters, unless you're building your query with dynamic SQL so that the query ends up getting executed with a literal.

A good article on the topic is Filtered Indexes and Dynamic SQL by Jeremiah Peschka. 

OR

As Martin suggested, you could add WITH (RECOMPILE) to the query, but that should not be used without understanding the potential repercussions (see RECOMPILE Hints and Execution Plan Caching).