sql server 'in' or 'or' - which is fastest

Question

Ok, so I have a query:

select distinct(a)
from mytable
where
b in (0,3)

What is going to be faster, the above or

select di         


        

Answer 1

Both IN and OR will do a query for b = 0 followed by one for b = 3, and then do a merge join on the two result sets, and finally filter out any duplicates.

With IN, duplicates doesn't really make sense, because b can't both be 0 and 3, but the fact is that IN will be converted to b = 0 OR b = 3, and with OR, duplicates do make sense, because you could have b = 0 OR a = 3, and if you were to join the two separate result sets, you could end up with duplicates for each record that matched both criteria.

So a duplicate filtering will always be done, regardless of whether you're using IN or OR. However, if you know from the outset that you will not have any duplicates - which is usually the case when you're using IN - then you can gain some performance by using UNION ALL which doesn't filter out duplicates:

select distinct(a)
from mytable
where
b = 0

UNION ALL

select distinct(a)
from mytable
where
b = 3