Question

Say, I have the below two tables

 ------------------------------
|           employee           |
 ------------------------------
| employee_id  | employee_name |  
 ------------------------------
|     1        |     one       |
|     2        |     two       |
|     3        |     three     |
 ------------------------------

and

 -------------------------------------------
|                feedback                   |
 -------------------------------------------
| employee_id (FK)  |       comments        |  
 -------------------------------------------
|         2         |     comment two       |
 -------------------------------------------

What is the best (in terms of performance) way of retrieving all employees who have not had their feedback given?

I was thinking of the below SQL but because it uses subquery, I am not sure how fast it will be when the number of records in both the tables grow.

SELECT * FROM employee WHERE employee_id NOT IN (SELECT employee_id FROM feedback)

The database is Oracle and all key columns have indexes.

Update

Thanks everybody, I wish I could accept more than one answer! This is what I used in the end (my table structure wasn’t quite as simple as shown here as I had joins with several other tables).

SELECT 
    e.name, m.name, a.postcode 
FROM 
    employee LEFT OUTER JOIN feedback f on (e.employee_id = f.employee_id),
    address a, manager m 
WHERE a.address_id = e.address_id
AND m.manager_id = e.manager_id
AND f.employee_id IS NULL

Answer 1

You could use:

SELECT e.* 
  FROM employee e 
  LEFT OUTER JOIN feedback f ON (e.employee_id = f.employee_id)
 WHERE f.employee_id IS NULL 

Which should be pretty good. I assume the EMPLOYEE_ID columns are indexed…

Try it and see what your explain plan looks like.

EDIT: As you have said you do not have the PLAN table then this article from Tom Kyte (Oracle VP) is useful:
http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:313616750808

It suggests reasoning behind why each solution (NOT IN, NOT EXISTS, OUTER JOIN) might be better under circumstances.

There is also this from the prolific Don Burleson:
http://www.dba-oracle.com/oracle_tips_subq_rewrite.htm