Suppose We have two Tables
Customers and Orders
orderid custid date
-------- ------ -----
101 1 2016-03-01
102 1 2016-03-03
103 2 2016-03-01
where custid not in
(Select custid from Orders)
from Customers C left join Orders O
on C.custid = O.custid
where O.orderid is null
Both queries are different,so comparison is not valid ..for the queries to be same/to get same execution plan..
Not IN in first query with
if you do this,you may get same plan for both queries and same treatment...
Not IN asked to be removed,other than the reason it may give you weird results when nulls are involved,can be found from this answer Not IN vs Not Exists
Pasting Relevant terms from answer..
The execution plans may be the same at the moment but if either column is altered in the future to allow NULLs the NOT IN version will need to do more work (even if no NULLs are actually present in the data) and the semantics of NOT IN if NULLs are present are unlikely to be the ones you want anyway.