Oracle not using the stored outline

Question

SQL> create table emp as select * from sys.emp;
Table created.
SQL> alter session set create_stored_outlines = TRUE;
Session altered.
SQL> create outline emp_dept for category scott_outlines on select empno from emp where ename = 'SCOTT';
Outline created.
SQL> set autot on exp
SQL> select empno from emp where ename = 'SCOTT';
EMPNO
7788
Execution Plan
Plan hash value: 3956160932
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | 1 | 20 | 2 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| EMP | 1 | 20 | 2 (0)| 00:00:01 |
Predicate Information (identified by operation id):
1 - filter("ENAME"='SCOTT')
Note
- dynamic sampling used for this statement (level=2)
SQL> create unique index i on emp(ename);
Index created.
SQL> select empno from emp where ename = 'SCOTT';
EMPNO
7788
Execution Plan
Plan hash value: 3262377121
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
|
| 0 | SELECT STATEMENT | | 1 | 20 | 1 (0)| 00:00:
01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 20 | 1 (0)| 00:00:
01 |
|* 2 | INDEX UNIQUE SCAN | I | 1 | | 0 (0)| 00:00:
01 |
Predicate Information (identified by operation id):
2 - access("ENAME"='SCOTT')
SQL> alter session set use_stored_outlines = SCOTT_OUTLIN
2 ;
Session altered.
SQL> alter session set use_stored_outlines = SCOTT_OUTLINS
2 ;
Session altered.
SQL> select empno from emp where ename = 'SCOTT';
EMPNO
7788
Execution Plan
Plan hash value: 3262377121
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time
|
| 0 | SELECT STATEMENT | | 1 | 20 | 1 (0)| 00:00:
01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 20 | 1 (0)| 00:00:
01 |
|* 2 | INDEX UNIQUE SCAN | I | 1 | | 0 (0)| 00:00:
01 |
Predicate Information (identified by operation id):
2 - access("ENAME"='SCOTT')
Note
- outline "SYS_OUTLINE_11050409142489113" used for this statement
SQL> SELECT name, category, used FROM user_outlines;
NAME CATEGORY USED
EMP_DEPT SCOTT_OUTLINES UNUSED
SYS_OUTLINE_11050408594412502 DEFAULT USED
SYS_OUTLINE_11050408591781301 DEFAULT UNUSED
SYS_OUTLINE_11050408594415603 DEFAULT UNUSED
SYS_OUTLINE_11050408595648404 DEFAULT UNUSED
SYS_OUTLINE_11050409003554705 DEFAULT UNUSED
SYS_OUTLINE_11050409030340606 DEFAULT UNUSED
7 rows selected.
Execution Plan
Plan hash value: 1195863419
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Ti
me |
| 0 | SELECT STATEMENT | | 1 | 81 | 2 (0)| 00
:00:01 |
| 1 | NESTED LOOPS | | | | |
|
| 2 | NESTED LOOPS | | 1 | 81 | 2 (0)| 00
:00:01 |
| 3 | TABLE ACCESS FULL | OL$ | 1 | 64 | 2 (0)| 00
:00:01 |
|* 4 | INDEX UNIQUE SCAN | I_USER1 | 1 | | 0 (0)| 00
:00:01 |
|* 5 | TABLE ACCESS BY INDEX ROWID| USER$ | 1 | 17 | 0 (0)| 00
:00:01 |
Predicate Information (identified by operation id):
4 - access("CREATOR"="U"."NAME")
5 - filter("U"."USER#"=USERENV('SCHEMAID'))
Note
- outline "SYS_OUTLINE_11050409030340606" used for this statement
SQL>
Note : I have dropped all default outlines in dba_outlines but they are being created automatically.(Why)
Please give me good article to understand more on stored outlines.

Answer 1

Please post your 4 digits Oracle version.
It looks like that Oracle has only 1 stored outline for the SQL statement and that the second execution plan with the index has replaced the first outline (execution plan with full table scan).
Here is a good article on stored outlines http://www.oracle-base.com/articles/misc/Outlines.php (except that the used Oracle version is also missing).
Here is a short demo based on your demo that I have modified (note that I disabled stored outlines just after creating the first one):
SQL> select * from v$version;
BANNER
Oracle Database 10g Express Edition Release 10.2.0.1.0 - Product
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
SQL> drop table emp purge;
Table dropped.
SQL> drop outline emp_dept;
Outline dropped.
SQL> whenever sqlerror exit failure;
SQL> --
SQL> create table emp as
  2  select object_name ename, object_id empno
  3  from all_objects
  4  where object_id < 5000;
Table created.
SQL> --
SQL> alter session set create_stored_outlines = TRUE;
Session altered.
SQL> create outline emp_dept for category scott_outlines on
select empno from emp where ename = 'SCOTT';
Outline created.
SQL> set autot on exp
SQL> select empno from emp where ename = 'SCOTT';
no rows selected
Execution Plan
Plan hash value: 3956160932
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |      |     1 |    30 |     5   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EMP  |     1 |    30 |     5   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("ENAME"='SCOTT')
Note
   - dynamic sampling used for this statement
SQL> -- disable stored outline creation
SQL> alter session set create_stored_outlines = FALSE;
Session altered.
SQL> create index i on emp(ename);
Index created.
SQL> select empno from emp where ename = 'SCOTT';
no rows selected
Execution Plan
Plan hash value: 4079916893
| Id  | Operation                   | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |      |     1 |    30 |     1   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP  |     1 |    30 |     1   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | I    |     1 |       |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - access("ENAME"='SCOTT')
Note
   - dynamic sampling used for this statement
SQL> -- use stored outlines
SQL> alter session set use_stored_outlines = SCOTT_OUTLINES;
Session altered.
SQL> select empno from emp where ename = 'SCOTT';
no rows selected
Execution Plan
Plan hash value: 3956160932
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |      |    10 |   300 |     5   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EMP  |    10 |   300 |     5   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("ENAME"='SCOTT')
Note
   - outline "EMP_DEPT" used for this statement
SQL> -- do not use stored outlines
SQL> alter session set use_stored_outlines=false;
Session altered.
SQL> select empno from emp where ename = 'SCOTT';
no rows selected
Execution Plan
Plan hash value: 4079916893
| Id  | Operation                   | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT            |      |     1 |    30 |     1   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP  |     1 |    30 |     1   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | I    |     1 |       |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   2 - access("ENAME"='SCOTT')
Note
   - dynamic sampling used for this statementEdited by: P. Forstmann on 4 mai 2011 13:34