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Avoid Sorting done by 'ORDER BY' clause
Sorting is mostly expensive, if you have an index, you can use it to avoid
sorting ! Is NOT NULL constraint required? The following TEST CASES will answer
your question.
TEST CASE 1:
Step 1: Create a table EMP as
SQL> conn system/password as sysdba;
Connected.
SQL> CREATE TABLE HASAN.EMP AS SELECT * FROM SCOTT.EMP;
Table created.
SQL> conn hasan/password
Connected. |
Step 2: Query the EMP
SQL> set autotrace on explain;
SQL> select * from EMP ORDER BY EMPNO;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ----------
----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2007178810
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1218 | 4 (25) | 00:00:01 |
| 1 | SORT ORDER BY | | 14 | 1218 | 4 (25) | 00:00:01 |
| 2 | TABLE ACCESS FULL| EMP | 14 | 1218 | 3 (0) | 00:00:01 |
--------------------------------------------------------------------------- |
Observation: Full Table Scan of EMP and SORTING IS DOING WITH EMPNO
Step 3: Create a index TESTEMPIDX on EMPNO OF TESTEMP table as
SQL> create index emp_empno_indx on emp(empno);
Index created.
SQL> select * from emp order by empno;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ----------
----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 150391907
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1218 | 4 (25)| 00:00:01 |
| 1 | SORT ORDER BY | | 14 | 1218 | 4 (25)| 00:00:01 |
| 2 | TABLE ACCESS FULL| EMP | 14 | 1218 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------- |
Observation: Full Table Scan of EMP and SORTING (ORDERBY)
Step 4: Create a NOT NULL Constraint on EMPNO of EMP table as
SQL> ALTER TABLE EMP MODIFY (EMPNO CONSTRAINT NOT_NULL_CONS NOT NULL);
Table altered.
SQL> desc emp;
Name Null? Type
----------------- -------- ------------
EMPNO NOT NULL NUMBER(4)
ENAME VARCHAR2(10)
JOB VARCHAR2(9)
MGR NUMBER(4)
HIREDATE DATE
SAL NUMBER(7,2)
COMM NUMBER(7,2)
DEPTNO NUMBER(2)
SQL> select * from emp order by empno;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ----------
----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1969959806
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1218 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 14 | 1218 | 2 (0)| 00:00:01 |
| 2 | INDEX FULL SCAN | EMP_EMPNO_INDX | 14 | | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------- |
Observation: INDEX TESTEMPIDX of TESTEMP is USING, TABLE ACCESS BY
INDEX ROWID AND NOSORTING
Learning:
1) Order by column should have NOT NULL constraint.
2) Index column should be in the Order by Clause
TEST CASE 2 :
SQL> desc emp;
Name Null? Type
----------------- -------- ------------
EMPNO NOT NULL NUMBER(4)
ENAME VARCHAR2(10)
JOB VARCHAR2(9)
MGR NUMBER(4)
HIREDATE DATE
SAL NUMBER(7,2)
COMM NUMBER(7,2)
DEPTNO NUMBER(2)
SQL> select index_name,column_name from user_ind_columns where table_name='EMP';
INDEX_NAME COLUMN_NAME
------------- --------------
EMP_EMPNO_INDX EMPNO |
Step 1: QUERY the EMP as
SQL> SELECT * FROM EMP ORDER BY ENAME;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ----------
----------
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7934 MILLER CLERK 7782 23-JAN-82 1300 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7369 SMITH CLERK 7902 17-DEC-80 800 20
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 150391907
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1218 | 4 (25)| 00:00:01 |
| 1 | SORT ORDER BY | | 14 | 1218 | 4 (25)| 00:00:01 |
| 2 | TABLE ACCESS FULL| EMP | 14 | 1218 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------- |
Observation: After adding ENAME in Order by clause, index TESTEMPIDX
is not using
Step 2: Create a Composite index on EMPNO and ENAME and run a SQL again as
SQL> CREATE INDEX TESTEMPIDX2 ON EMP(EMPNO,ENAME);
Index created.
SQL> SELECT * FROM EMP ORDER BY EMPNO,ENAME;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ----------
----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1356242066
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1218 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 14 | 1218 | 2 (0)| 00:00:01 |
| 2 | INDEX FULL SCAN | TESTEMPIDX2 | 14 | | 1 (0)| 00:00:01 |
--------------------------------------------------------------------------------------- |
Observation: INDEX TESTEMPIDX2 of EMP is USING, TABLE ACCESS BY INDEX
ROWID AND NOSORTING
Step 3: alternate the order of EMPNO and ENAME and run a SQL again as
SQL> SELECT * FROM EMP ORDER BY ENAME,EMPNO;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ----------
----------
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7934 MILLER CLERK 7782 23-JAN-82 1300 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7369 SMITH CLERK 7902 17-DEC-80 800 20
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 150391907
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1218 | 4 (25)| 00:00:01 |
| 1 | SORT ORDER BY | | 14 | 1218 | 4 (25)| 00:00:01 |
| 2 | TABLE ACCESS FULL| EMP | 14 | 1218 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------- |
Observation: Full Table Scan of EMP and SORTING (ORDER BY) (THE INDEX
TESTEMPIDX2 is not used !)
Conclusion:
1. All the Order by column should be in composite index
2. Order by columns should be of the same order as of composite index
3. At least one of the composite columns should have NOT NULL constraint.
Environment:
Operating System: Windows
Database: Oracle 10g R2
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