오라클 성능 고도화 원리와 해법 II (2012년)
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인덱스를 이용한 소트 연산 대체 0 0 60,977

by 구루비스터디 소트튜닝 Sort Order By Sort Group By [2018.04.01]

  1. 인덱스를 이용한 소트 연산 대체
    1. PK 컬럼에 distinct 쿼리 수행


인덱스를 이용한 소트 연산 대체

  • 인덱스는 항상 키 컬럼 순으로 정렬된 상태 유지 : sort 연산 생략 가능


PK 컬럼에 distinct 쿼리 수행


특징
  • sort unique 명령 생략 = sort unique nosort
  • sort 수행 없이 인덱스를 이용해 unique한 집합 출력

SQL> SELECT DISTINCT empno FROM emp;

Execution Plan
----------------------------------------------------------
Plan hash value: 179099197

---------------------------------------------------------------------------
| Id  | Operation        | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT |        |    14 |    56 |     1   (0)| 00:00:01 |
|   1 |  INDEX FULL SCAN | PK_EMP |    14 |    56 |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------

SQL> SELECT DISTINCT empno FROM emp ORDER BY empno;

Execution Plan
----------------------------------------------------------
Plan hash value: 179099197

---------------------------------------------------------------------------
| Id  | Operation        | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT |        |    14 |    56 |     1   (0)| 00:00:01 |
|   1 |  INDEX FULL SCAN | PK_EMP |    14 |    56 |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------

SQL> CREATE TABLE emp1 AS SELECT * FROM emp;

테이블이 생성되었습니다.

SQL> CREATE INDEX idx_emp1 ON emp1(empno);

인덱스가 생성되었습니다.

SQL> SELECT DISTINCT empno FROM emp1;

Execution Plan
----------------------------------------------------------
Plan hash value: 3282348538

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |    14 |   182 |     4  (25)| 00:00:01 |
|   1 |  HASH UNIQUE       |      |    14 |   182 |     4  (25)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMP1 |    14 |   182 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL> SELECT DISTINCT empno FROM emp1 ORDER BY empno;

Execution Plan
----------------------------------------------------------
Plan hash value: 2849225206

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |    14 |   182 |     5  (40)| 00:00:01 |
|   1 |  SORT UNIQUE       |      |    14 |   182 |     4  (25)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMP1 |    14 |   182 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


sort order by 대체
  • 특징
    • 정렬해야 할 대상 레코드는 무수히 많고, 그 중 일부만 읽고 멈출 수 있는 업무에서만 유리
    • 인덱스를 스캔하면서 결과집합을 끝까지 fetch한다면 오히려 I/O 및 리소스 사용 측면에서 손해.
    • 대상 레코드가 소량일 때는 정렬이 발생하더라도 부하가 크지 않아 개선 효과도 미미함

SQL> CREATE TABLE customer
  2  AS
  3  SELECT LEVEL custid
  4       , CHR(64 + CEIL(LEVEL / 100)) region
  5       , dbms_random.string('U', 4) name
  6       , ROUND(dbms_random.value(10, 70)) age
  7    FROM dual
  8   CONNECT BY LEVEL <= 1000
  9  ;

테이블이 생성되었습니다.

SQL> CREATE INDEX customer_x01 ON customer(region);

인덱스가 생성되었습니다.

SQL> SELECT *
  2    FROM customer
  3   WHERE region = 'A'
  4   ORDER BY custid
  5  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 3806818772

--------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |   100 |   198K|     3  (34)| 00:00:01
|   1 |  SORT ORDER BY               |              |   100 |   198K|     3  (34)| 00:00:01
|   2 |   TABLE ACCESS BY INDEX ROWID| CUSTOMER     |   100 |   198K|     2   (0)| 00:00:01
|*  3 |    INDEX RANGE SCAN          | CUSTOMER_X01 |   100 |       |     1   (0)| 00:00:01
--------------------------------------------------------------------------------------------

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

   3 - access("REGION"='A')

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL> CREATE INDEX customer_x02 ON customer(region, custid);

인덱스가 생성되었습니다.

SQL> SELECT *
  2    FROM customer
  3   WHERE region = 'A'
  4   ORDER BY custid
  5  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 2477059019

--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |   100 |   198K|     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| CUSTOMER     |   100 |   198K|     3   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | CUSTOMER_X02 |   100 |       |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

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

   2 - access("REGION"='A')

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL>


sort group by 대체
  • 특징
    • sort group by nosort
    • 인덱스를 이용한 nosort 방식으로 수행될 때는 group by 명령에도 불구하고 부분범위 처리가 가능해져 성능 개선 가능




SQL> SELECT region
  2       , AVG(age) age
  3       , COUNT(*) cnt
  4    FROM customer
  5   GROUP BY region
  6  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 3656427734

-------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |  1000 | 16000 |     4  (25)| 00:00:01 |
|   1 |  HASH GROUP BY     |          |  1000 | 16000 |     4  (25)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| CUSTOMER |  1000 | 16000 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL> SELECT region
  2       , AVG(age) age
  3       , COUNT(*) cnt
  4    FROM customer
  5   GROUP BY region
  6   ORDER BY region
  7  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 3481805491

-------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |  1000 | 16000 |     4  (25)| 00:00:01 |
|   1 |  SORT GROUP BY     |          |  1000 | 16000 |     4  (25)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| CUSTOMER |  1000 | 16000 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL> SELECT region
  2       , AVG(age) age
  3       , COUNT(*) cnt
  4    FROM customer
  5   WHERE region IS NOT NULL
  6   GROUP BY region
  7  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 3656427734

-------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |  1000 | 16000 |     4  (25)| 00:00:01 |
|   1 |  HASH GROUP BY     |          |  1000 | 16000 |     4  (25)| 00:00:01 |
|*  2 |   TABLE ACCESS FULL| CUSTOMER |  1000 | 16000 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------

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

   2 - filter("REGION" IS NOT NULL)

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL> SELECT region
  2       , AVG(age) age
  3       , COUNT(*) cnt
  4    FROM customer
  5   WHERE region IS NOT NULL
  6   GROUP BY region
  7   ORDER BY region
  8  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 3481805491

-------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |  1000 | 16000 |     4  (25)| 00:00:01 |
|   1 |  SORT GROUP BY     |          |  1000 | 16000 |     4  (25)| 00:00:01 |
|*  2 |   TABLE ACCESS FULL| CUSTOMER |  1000 | 16000 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------

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

   2 - filter("REGION" IS NOT NULL)

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL> SELECT /*+ first_rows(10) */ region
  2       , AVG(age) age
  3       , COUNT(*) cnt
  4    FROM customer
  5   WHERE region IS NOT NULL
  6   GROUP BY region
  7  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 89828339

---------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |  1000 | 16000 |     3   (0)| 00:00:01 |
|   1 |  SORT GROUP BY NOSORT        |              |  1000 | 16000 |     3   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| CUSTOMER     |  1000 | 16000 |     3   (0)| 00:00:01 |
|*  3 |    INDEX FULL SCAN           | CUSTOMER_X02 |     1 |       |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

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

   3 - filter("REGION" IS NOT NULL)

Note
-----
   - dynamic sampling used for this statement (level=2)


인덱스가 Sort 연산을 대체하지 못하는 경우
  • 옵티마이져모드가 all_rows 인 경우 풀스캔 가능성이 더 커진다. 풀스캔시 Sort 연산 수행
  • 옵티마이져모드가 first_rows 인 경우 인덱스스캔 가능성이 더 커진다. 인덱스 스캔시 Sort 연산 대체
  • 인덱스 항목이 NOT NULL 이 아닌 경우 : 인덱스만으로 모든 행을 가져오지 못하므로 인덱스 못탐(널 행 제외)
  • 결합인덱스의 경우엔 널값도 저장된다(순서상 맨 아래쪽에 저장) 따라서 다음의 경우 Sort 연산 대체 안됨
  • ORDER BY 컬럼 NULLS FIRST
  • ORDER BY 컬럼 DESC NULLS LAST



SQL> CREATE INDEX emp_deptno_ename_idx ON emp(deptno, ename);

인덱스가 생성되었습니다.

SQL> SELECT *
  2    FROM emp
  3   WHERE deptno = 30
  4   ORDER BY ename
  5  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 3268462453

----------------------------------------------------------------------------------------------------
| Id  | Operation                   | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                      |     5 |   190 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP                  |     5 |   190 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | EMP_DEPTNO_ENAME_IDX |     5 |       |     1   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------

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

   2 - access("DEPTNO"=30)

SQL> SELECT *
  2    FROM emp
  3   WHERE deptno = 30
  4   ORDER BY ename NULLS FIRST
  5  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 4293037890

-----------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                      |     5 |   190 |     3  (34)| 00:00:01 |
|   1 |  SORT ORDER BY               |                      |     5 |   190 |     3  (34)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| EMP                  |     5 |   190 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | EMP_DEPTNO_ENAME_IDX |     5 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------

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

   3 - access("DEPTNO"=30)

SQL> SELECT *
  2    FROM emp
  3   WHERE deptno = 30
  4   ORDER BY ename NULLS LAST
  5  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 3268462453

----------------------------------------------------------------------------------------------------
| Id  | Operation                   | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                      |     5 |   190 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP                  |     5 |   190 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | EMP_DEPTNO_ENAME_IDX |     5 |       |     1   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------

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

   2 - access("DEPTNO"=30)

SQL> SELECT *
  2    FROM emp
  3   WHERE deptno = 30
  4   ORDER BY ename DESC
  5  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 3122309019

-----------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                      |     5 |   190 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID | EMP                  |     5 |   190 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN DESCENDING| EMP_DEPTNO_ENAME_IDX |     5 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------

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

   2 - access("DEPTNO"=30)

SQL> SELECT *
  2    FROM emp
  3   WHERE deptno = 30
  4   ORDER BY ename DESC NULLS LAST
  5  ;

Execution Plan
----------------------------------------------------------
Plan hash value: 4293037890

-----------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                      |     5 |   190 |     3  (34)| 00:00:01 |
|   1 |  SORT ORDER BY               |                      |     5 |   190 |     3  (34)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| EMP                  |     5 |   190 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | EMP_DEPTNO_ENAME_IDX |     5 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------

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

   3 - access("DEPTNO"=30)
"구루비 데이터베이스 스터디모임" 에서 2012년에 "오라클 성능 고도화 원리와 해법 II " 도서를 스터디하면서 정리한 내용 입니다.

- 강좌 URL : http://www.gurubee.net/lecture/3301

- 구루비 강좌는 개인의 학습용으로만 사용 할 수 있으며, 다른 웹 페이지에 게재할 경우에는 출처를 꼭 밝혀 주시면 고맙겠습니다.~^^

- 구루비 강좌는 서비스 제공을 위한 목적이나, 학원 홍보, 수익을 얻기 위한 용도로 사용 할 수 없습니다.

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