SQL Server에서는 IS NULL 조건자와 함께 작성 하는 필터링 된 인덱스를 사용 하지 않습니다. 필터드 인덱스 사용시 IS NULL 조건자의 경우 include에 컬럼 추가되어야 함

https://support.microsoft.com/ko-kr/help/3051225/a-filtered-index-that-you-create-together-with-the-is-null-predicate-i

예) 필터드 인덱스 조건 중 IS NULL의 경우 include 컬럼 항목

CREATE NONCLUSTERED INDEX New_i_action_filt_action_date_type ON dbo.filter_test (action_type) include (action_date) WHERE action_date IS NULL