UNION ALL vs OR condition in sql server query

Question

I have to select some rows based on a not exists condition on a table. If I use a union all as below, it gets executed in less than 1 second.

SELECT 1 FROM d         


        

Answer 1

The usage of the OR is probably causing the query optimizer to no longer use an index in the second query. Look at the explain for each query and that will tell you the answer.