What is faster?

Question

Hi Gurus,
can you please suggest me what is right way?
1. to join three tables in one SELECT
or
2. to have three SELECT's
In reality first query return ID for second query and this will return also another ID for "real" third query.

Answer 1

All good points - however just to nitpick your IN vs EXISTS example, the optimizer will generally transform both queries into a semijoin using an index, if that will result in a lower cost:
SQL> @ind dept
                                                                                  Distinct
Index                        Unique? Type                  Status          Rows       keys Column name
PK_DEPT                      Y       NORMAL                VALID              4          4 DEPTNO
SQL> @ind emp
                                                                                  Distinct
Index                        Unique? Type                  Status          Rows       keys Column name
PK_EMP                       Y       NORMAL                VALID             14         14 EMPNO
SQL> SELECT dname, deptno
  2  FROM   dept d
  3  WHERE  d.deptno IN (SELECT e.deptno FROM emp e)
  4 
SQL> @xplan
Plan hash value: 1090737117
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |         |     3 |    48 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN SEMI             |         |     3 |    48 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    52 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT UNIQUE                |         |    14 |    42 |     4  (25)| 00:00:01 |
|   5 |    TABLE ACCESS FULL         | EMP     |    14 |    42 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   4 - access("D"."DEPTNO"="E"."DEPTNO")
       filter("D"."DEPTNO"="E"."DEPTNO")
SQL> SELECT dname, deptno
  2  FROM   dept d
  3  WHERE  EXISTS
  4         ( SELECT e.deptno FROM emp e
  5           WHERE  d.deptno = e.deptno)
  6 
SQL> @xplan
Plan hash value: 1090737117
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |         |     3 |    48 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN SEMI             |         |     3 |    48 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    52 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT UNIQUE                |         |    14 |    42 |     4  (25)| 00:00:01 |
|   5 |    TABLE ACCESS FULL         | EMP     |    14 |    42 |     3   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   4 - access("E"."DEPTNO"="D"."DEPTNO")
       filter("E"."DEPTNO"="D"."DEPTNO")
SQL> create index emp_deptno_ix on emp(deptno) compute statistics;
Index created.
SQL> SELECT dname, deptno
  2  FROM   dept d
  3  WHERE  d.deptno IN (SELECT e.deptno FROM emp e)
  4 
SQL> @xplan
Plan hash value: 151962099
| Id  | Operation          | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |               |     3 |    48 |     3   (0)| 00:00:01 |
|   1 |  NESTED LOOPS SEMI |               |     3 |    48 |     3   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| DEPT          |     4 |    52 |     3   (0)| 00:00:01 |
|*  3 |   INDEX RANGE SCAN | EMP_DEPTNO_IX |     9 |    27 |     0   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   3 - access("D"."DEPTNO"="E"."DEPTNO")I just mentioned this because "EXISTS is faster than IN" is often repeated around the forums when it is not necessarily true. Of course, I totally agree with the general point, that an SQL query can be written in many different ways and some are far less efficient than others (and cannot be automatically transformed into efficient ones by the optimizer).