15

I have a table with a unique index filtered for non-nullable values. In the query plan there is use of distinct. Is there a reason for this?

USE tempdb

CREATE TABLE T1( Id INT NOT NULL  IDENTITY PRIMARY KEY ,F1 INT , F2 INT )
go
CREATE UNIQUE NONCLUSTERED INDEX UK_T1 ON T1 (F1,F2) WHERE F1 IS NOT NULL AND F2 IS NOT NULL 
GO
INSERT INTO  T1(f1,F2) VALUES(1,1),(1,2),(2,1)

SELECT DISTINCT   F1,F2 FROM T1 WHERE F1 IS NOT NULL AND F2 IS NOT NULL 
SELECT  F1,F2 FROM T1 WHERE F1 IS NOT NULL AND F2 IS NOT NULL  

query plan : enter image description here

migrated from stackoverflow.com Aug 7 '14 at 11:14

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15

This is a known SQL Server query optimizer limitation. It has been reported to Microsoft in this Connect item. There are additional consequences of this limitation, including some that I wrote about in this article.

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