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9. Outer 조인을 Inner 조인으로 변환




Outer 조인을 Inner 조인으로 변환

: Outer 조인문을 작성하면 일부 조건절에 Outer 기호를 빠뜨리면 Inner 조인할 때와 같은 결과가 나온다.
이럴때 옵티마이저는 Outer 조인을 Inner 조인문으로 바꾸는 쿼리 변환을 시행하게 된다.

 

select *
    from   emp e, dept d
    where  d.deptno(+) = e.deptno
    and    d.loc = 'DALLAS'
    and    e.sal >= 1000;

(10g)
Execution Plan
----------------------------------------------------------
Plan hash value: 3493486646

------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                |     5 |   285 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS BY INDEX ROWID  | EMP            |     5 |   185 |     1   (0)| 00:00:01 |
|   2 |   NESTED LOOPS                |                |     5 |   285 |     3   (0)| 00:00:01 |
|   3 |    TABLE ACCESS BY INDEX ROWID| DEPT           |     1 |    20 |     2   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | DEPT_IDX       |     1 |       |     1   (0)| 00:00:01 |
|*  5 |    INDEX RANGE SCAN           | EMP_DEPTNO_IDX |     5 |       |     0   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("E"."SAL">=1000)
   4 - access("D"."LOC"='DALLAS')
   5 - access("D"."DEPTNO"="E"."DEPTNO")



(11g)

Execution Plan
----------------------------------------------------------
Plan hash value: 844388907

----------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |     4 |   232 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN                  |         |     4 |   232 |     6  (17)| 00:00:01 |
|*  2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     1 |    20 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT JOIN                  |         |    12 |   456 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMP     |    12 |   456 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("D"."LOC"='DALLAS')
   4 - access("D"."DEPTNO"="E"."DEPTNO")
       filter("D"."DEPTNO"="E"."DEPTNO")
   5 - filter("E"."SAL">=1000)


-옵티마이저가 쿼리 변환을 시행하는 이유는 조인 순서를 자유롭게 결정하기 위함
-Outer NL조인 / Outer 소트머지 조인시 드라이빙 테이블은 항상 Outer 기호가 붙지 않은 쪽으로 고정됨.
-Outer 해시 조인시 자유롭게 조인순서가 바뀌도록 개선됨(10g부터)

만약 위의 쿼리에서 sal >=1000 조건에 부합하는 사원 레코드가 매우 많고, loc='DALLAS' 조건에 부합하는 부서에 속한 사원이 매우 적다면 dept 테이블을 먼저 드라이빙하는 것이 유리하다. 그럼에도 Outer 조인 때문에 항상 emp테이블을 드라이빙 해야 한다면 불리한 조건에서 최적화하는 것이 된다. 이러한 이유로 불필요한 Outer 조인을 삼가 해야한다.

Outer 조인을 써야하는 상황이라면 Outer 기호를 정확히 구사해야 올바른 결과 집합을 얻을 수 있음에 유념하자
ANSI Outer 조인문일때는 Outer 기호 대신 조건절 위치에 신경써야 한다.

Outer 조인에서 Inner 쪽 테이블에 대한 필터 조건을 아래처럼 where 절에 기술한다면 Inner 조인할 때와 같은 결과 집합을 얻게 된다. 따라서 옵티마이저가 Outer 조인을 Inner 조인으로 변환해버린다.


select e.empno, e.deptno, e.sal, d.loc, d.dname, d.deptno
from dept d left outer join emp e on d.deptno = e.deptno
where e.sal > 1000;

(10g)책
Execution Plan
----------------------------------------------------------
Plan hash value: 3582342135

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |             |    13 |   403 |     3   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |             |    13 |   403 |     3   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| EMP         |    13 |   143 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | EMP_SAL_IDX |    13 |       |     1   (0)| 00:00:01 |
|   4 |   TABLE ACCESS BY INDEX ROWID| DEPT        |     1 |    20 |     1   (0)| 00:00:01 |
|*  5 |    INDEX UNIQUE SCAN         | DEPT_PK     |     1 |       |     0   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("E"."SAL">1000)
   5 - access("D"."DEPTNO"="E"."DEPTNO")

(10g)집
Execution Plan
----------------------------------------------------------
Plan hash value: 844388907

----------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |    13 |   403 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN                  |         |    13 |   403 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    80 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT JOIN                  |         |    13 |   143 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMP     |    13 |   143 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   4 - access("D"."DEPTNO"="E"."DEPTNO")
       filter("D"."DEPTNO"="E"."DEPTNO")
   5 - filter("E"."SAL">1000)

(11g)
Execution Plan
----------------------------------------------------------
Plan hash value: 844388907

----------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |    12 |   372 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN                  |         |    12 |   372 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    80 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT JOIN                  |         |    12 |   132 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMP     |    12 |   132 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   4 - access("D"."DEPTNO"="E"."DEPTNO")
       filter("D"."DEPTNO"="E"."DEPTNO")
   5 - filter("E"."SAL">1000)


제대로된 Outer 조인 결과 집합을 얻으려면 sal>1000조건을 아래와 같이 on 절에 추가해줘야 한다.


 select e.empno, e.deptno, e.sal, d.loc, d.dname, d.deptno
 from dept d left outer join emp e on d.deptno = e.deptno and e.sal > 1000

(10g)책
Execution Plan
----------------------------------------------------------
Plan hash value: 1350698460

-----------------------------------------------------------------------------------------------
| Id  | Operation                    | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                |    13 |   403 |     4   (0)| 00:00:01 |
|   1 |  NESTED LOOPS OUTER          |                |    13 |   403 |     4   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL          | DEPT           |     4 |    80 |     3   (0)| 00:00:01 |
|*  3 |   TABLE ACCESS BY INDEX ROWID| EMP            |     3 |    33 |     1   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN          | EMP_DEPTNO_IDX |     5 |       |     0   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("E"."SAL"(+)>1000)
   4 - access("D"."DEPTNO"="E"."DEPTNO"(+))

(10g) 집
Execution Plan
----------------------------------------------------------
Plan hash value: 2251696546

----------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |    13 |   403 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN OUTER            |         |    13 |   403 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    80 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT JOIN                  |         |    13 |   143 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMP     |    13 |   143 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   4 - access("D"."DEPTNO"="E"."DEPTNO"(+))
       filter("D"."DEPTNO"="E"."DEPTNO"(+))
   5 - filter("E"."SAL"(+)>1000)

(11g)
Execution Plan
----------------------------------------------------------
Plan hash value: 2251696546

----------------------------------------------------------------------------------------
| Id  | Operation                    | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |         |    12 |   372 |     6  (17)| 00:00:01 |
|   1 |  MERGE JOIN OUTER            |         |    12 |   372 |     6  (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    80 |     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  4 |   SORT JOIN                  |         |    12 |   132 |     4  (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMP     |    12 |   132 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   4 - access("D"."DEPTNO"="E"."DEPTNO"(+))
       filter("D"."DEPTNO"="E"."DEPTNO"(+))
   5 - filter("E"."SAL"(+)>1000)

ANSI Outer 조인문에서 where 절을 기술한 Inner 쪽 필터 조건이 의미 있게 사용되는 경우는 아래처럼 is null 조건을 체크하는 경우뿐이며, 조인에 실패하는 레코드를 찾고자 할때 흔히 사용되는 SQL이다

select e.empno, e.deptno, e.sal, d.loc, d.dname, d.deptno
from  dept d left outer join emp e on d.deptno = e.deptno
where e.empno is null;

(10g)
Execution Plan
----------------------------------------------------------
Plan hash value: 4106494745

------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                |    14 |   434 |     4   (0)| 00:00:01 |
|*  1 |  FILTER                       |                |       |       |            |          |
|   2 |   NESTED LOOPS OUTER          |                |    14 |   434 |     4   (0)| 00:00:01 |
|   3 |    TABLE ACCESS FULL          | DEPT           |     4 |    80 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS BY INDEX ROWID| EMP            |     4 |    44 |     1   (0)| 00:00:01 |
|*  5 |     INDEX RANGE SCAN          | EMP_DEPTNO_IDX |     5 |       |     0   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("E"."EMPNO" IS NULL)
   5 - access("D"."DEPTNO"="E"."DEPTNO"(+))

(10g) 집
Execution Plan
----------------------------------------------------------
Plan hash value: 457395871

-----------------------------------------------------------------------------------------
| Id  | Operation                     | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |         |    14 |   434 |     6  (17)| 00:00:01 |
|*  1 |  FILTER                       |         |       |       |            |          |
|   2 |   MERGE JOIN OUTER            |         |    14 |   434 |     6  (17)| 00:00:01 |
|   3 |    TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    80 |     2   (0)| 00:00:01 |
|   4 |     INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  5 |    SORT JOIN                  |         |    14 |   154 |     4  (25)| 00:00:01 |
|   6 |     TABLE ACCESS FULL         | EMP     |    14 |   154 |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("E"."EMPNO" IS NULL)
   5 - access("D"."DEPTNO"="E"."DEPTNO"(+))
       filter("D"."DEPTNO"="E"."DEPTNO"(+))

(11g)
Execution Plan
----------------------------------------------------------
Plan hash value: 457395871

-----------------------------------------------------------------------------------------
| Id  | Operation                     | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |         |     1 |    31 |     6  (17)| 00:00:01 |
|*  1 |  FILTER                       |         |       |       |            |          |
|   2 |   MERGE JOIN OUTER            |         |     1 |    31 |     6  (17)| 00:00:01 |
|   3 |    TABLE ACCESS BY INDEX ROWID| DEPT    |     4 |    80 |     2   (0)| 00:00:01 |
|   4 |     INDEX FULL SCAN           | PK_DEPT |     4 |       |     1   (0)| 00:00:01 |
|*  5 |    SORT JOIN                  |         |    14 |   154 |     4  (25)| 00:00:01 |
|   6 |     TABLE ACCESS FULL         | EMP     |    14 |   154 |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("E"."EMPNO" IS NULL)
   5 - access("D"."DEPTNO"="E"."DEPTNO"(+))
       filter("D"."DEPTNO"="E"."DEPTNO"(+))

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