FILTER access path..

Question

Hi ,
I have read a section in Performance Tuning e-book regarding the "Use of EXISTS versus IN for Subqueries".
In this section an example with its explain plan is posed....
SELECT
e.employee_id, e.first_name, e.last_name, e.salary
FROM employees e
WHERE EXISTS (SELECT 1 FROM orders o
WHERE e.employee_id = o.sales_rep_id
AND o.customer_id = 144);
ID OPERATION OPTIONS OBJECT_NAME OPT COST
0 SELECT STATEMENT CHO
1 FILTER
2 TABLE ACCESS FULL EMPLOYEES ANA 155
3 TABLE ACCESS BY INDEX ROWID ORDERS ANA 3
4 INDEX RANGE SCAN ORD_CUSTOMER_IX ANA 1As this type of access path is not described , at least in this section, can somebody describe when is this used....????
NOTE: I use Oracle10g. v2
Thank you....
Sim

Answer 1

It does say "The plan requires a full table scan of the employees table, returning many rows. Each of these rows is then filtered against the orders table (through an index)." The FILTER operation consists of working through one set of rows performing some test against it, in this case an index lookup.
I'm a little surprised at those examples though. In 11g (using the predefined "OE" demo schema) I get the same plan for both versions:
| Id  | Operation                      | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT               |                 |     3 |    90 |     5  (20)| 00:00:01 |
|   1 |  NESTED LOOPS                  |                 |       |       |            |          |
|   2 |   NESTED LOOPS                 |                 |     3 |    90 |     5  (20)| 00:00:01 |
|   3 |    SORT UNIQUE                 |                 |     3 |    21 |     2   (0)| 00:00:01 |
|*  4 |     TABLE ACCESS BY INDEX ROWID| ORDERS          |     3 |    21 |     2   (0)| 00:00:01 |
|*  5 |      INDEX RANGE SCAN          | ORD_CUSTOMER_IX |     5 |       |     1   (0)| 00:00:01 |
|*  6 |    INDEX UNIQUE SCAN           | EMP_EMP_ID_PK   |     1 |       |     0   (0)| 00:00:01 |
|   7 |   TABLE ACCESS BY INDEX ROWID  | EMPLOYEES       |     1 |    23 |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   4 - filter("O"."SALES_REP_ID" IS NOT NULL)
   5 - access("O"."CUSTOMER_ID"=144)
   6 - access("E"."EMPLOYEE_ID"="O"."SALES_REP_ID")I haven't tested in 10g but I would expect similar results. If you can use either IN or EXISTS without affecting the logic, so can the optimizer.