Thursday, April 10, 2008

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.

TASTCASE 1:

Step1: 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.

Step2: 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

Step3: 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)

Step4: 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

TESTCASE 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


Step1: 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

Step3: 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.
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