NL(Nested Loop) 조인
Nested Loop Join 의 개념
- 예시) USE_NL(각각 테이블에 어떤 컬럼에 인덱스를 이용 할것 인가? )
SELECT 고객.* ,주문.*
FROM 고객 -- 1) [고객]테이블은 어떤 컬럼에 인덱스가 있으면 좋은가?
JOIN 주문 -- 2) [주문]테이블은 어떤 컬럼에 인덱스가 있으면 좋은가?
ON 주문.고객번호 = 고객.고객번호
WHERE 고객.고객명='홍길동'
AND 주문.주문일자='201909';
파일:Use nl.jpg
USE_NL 조인
- 고객(outer)은 WHERE절의 = 조건(고객명) 인덱스 여부,
- 주문(inner)은 고객번호 컬럼 인덱스 여부가 N/L조인의 성능을 결정.
NL조인 시 인덱스 의 중요성
인덱스 오브젝트 장점
인덱스 조건
SELECT /*+ USE_NL(B) LEADING(A) */ ENAME,JOB,B.DNAME
FROM EMP A
JOIN DEPT B
ON A.DEPTNO = B.DEPTNO
-------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 17 (100)| | 14 |00:00:00.01 | 11 |
| 1 | NESTED LOOPS | | 1 | 14 | 672 | 17 (0)| 00:00:01 | 14 |00:00:00.01 | 11 |
| 2 | NESTED LOOPS | | 1 | 14 | 672 | 17 (0)| 00:00:01 | 14 |00:00:00.01 | 10 |
| 3 | TABLE ACCESS FULL | EMP | 1 | 14 | 364 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 7 |
|* 4 | INDEX RANGE SCAN | IX_DEPT_01 | 14 | 1 | | 0 (0)| | 14 |00:00:00.01 | 3 |
| 5 | TABLE ACCESS BY INDEX ROWID| DEPT | 14 | 1 | 22 | 1 (0)| 00:00:01 | 14 |00:00:00.01 | 1 |
-------------------------------------------------------------------------------------------------------------------------------------
LEADING 이나 ORDERED 힌트와 같이 사용 추천
- INDEX 구성
- EMP.IX_EMP_01 ( DEPTNO)
- DEPT.IX_DEPT_01 ( DEPTNO))
SELECT /*+ USE_NL(A,B) LEADING(A) */ ENAME,JOB,B.DNAME FROM EMP A JOIN DEPT B ON A.DEPTNO = B.DEPTNO ------------------------------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | ------------------------------------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | | | 17 (100)| | 14 |00:00:00.01 | 11 | | 1 | NESTED LOOPS | | 1 | 14 | 672 | 17 (0)| 00:00:01 | 14 |00:00:00.01 | 11 | | 2 | NESTED LOOPS | | 1 | 14 | 672 | 17 (0)| 00:00:01 | 14 |00:00:00.01 | 10 | | 3 | TABLE ACCESS FULL | EMP | 1 | 14 | 364 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 7 | |* 4 | INDEX RANGE SCAN | IX_DEPT_01 | 14 | 1 | | 0 (0)| | 14 |00:00:00.01 | 3 | | 5 | TABLE ACCESS BY INDEX ROWID| DEPT | 14 | 1 | 22 | 1 (0)| 00:00:01 | 14 |00:00:00.01 | 1 | -------------------------------------------------------------------------------------------------------------------------------------
USE_NL 괄호 안의 테이블은 NL조인 적용 대상 테이블
SELECT /*+ USE_NL(A,B) LEADING(B) */ ENAME,JOB,B.DNAME FROM EMP A JOIN DEPT B ON A.DEPTNO = B.DEPTNO ------------------------------------------------------------------------------------------------------------------------------------ | Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | ------------------------------------------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | | | 6 (100)| | 14 |00:00:00.01 | 10 | | 1 | NESTED LOOPS | | 1 | 14 | 672 | 6 (0)| 00:00:01 | 14 |00:00:00.01 | 10 | | 2 | NESTED LOOPS | | 1 | 20 | 672 | 6 (0)| 00:00:01 | 14 |00:00:00.01 | 9 | | 3 | TABLE ACCESS FULL | DEPT | 1 | 4 | 88 | 3 (0)| 00:00:01 | 4 |00:00:00.01 | 7 | |* 4 | INDEX RANGE SCAN | IX_EMP_01 | 4 | 5 | | 0 (0)| | 14 |00:00:00.01 | 2 | | 5 | TABLE ACCESS BY INDEX ROWID| EMP | 14 | 4 | 104 | 1 (0)| 00:00:01 | 14 |00:00:00.01 | 1 | ------------------------------------------------------------------------------------------------------------------------------------