Building virtual index using the NOSEGMENT clause.

Let’s create a virtual index

07:59:12 orcl> create index hr.emp2_emp_id_virtual on hr.employees2(employee_id) nosegment;
Index created.

I am going to set a hidden startup parameter “_use_nosegment_indexes” to TRUE so that our session will recognize our new virtual index.

08:00:09 orcl> alter session set “_use_nosegment_indexes” = true;
Running our statement again to see if it will use our new virtual index. Check out the access path below. The optimizer has chosen our virtual index.

select employee_id, a.department_id, b.department_name from
hr.departments b, hr.employees2 a where
a.department_id = b.department_id
and employee_id = 203

Execution Plan
———————————————————-
Plan hash value: 2516110069
—————————————————————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————————————-
| 0 | SELECT STATEMENT | | 1 | 25 | 3 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 1 | 25 | 3 (0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| EMPLOYEES2 | 1 | 9 | 2 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | EMP2_EMP_ID_VIRTUAL | 1 | | 1 (0)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| DEPARTMENTS | 1 | 16 | 1 (0)| 00:00:01 |
|* 5 | INDEX UNIQUE SCAN | DEPT_ID_PK | 1 | | 0 (0)| 00:00:01 |
—————————————————————————————————-

If I set the “_use_nosegment_indexes” to FALSE, the optimizer did NOT choose virtual index.

08:01:09 orcl> alter session set “_use_nosegment_indexes” = false;
Session altered.

08:01:33 orcl> select employee_id, a.department_id, b.department_name
08:01:47 2 from
08:01:47 3 hr.departments b, hr.employees2 a
08:01:47 4 where
08:01:47 5 a.department_id = b.department_id
08:01:47 6 and employee_id = 203;

Execution Plan
———————————————————-
Plan hash value: 2641883601
——————————————————————————————–

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
——————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 25 | 818 (3)| 00:00:10 |
| 1 | NESTED LOOPS | | 1 | 25 | 818 (3)| 00:00:10 |
|* 2 | TABLE ACCESS FULL | EMPLOYEES2 | 1 | 9 | 817 (3)| 00:00:10 |
| 3 | TABLE ACCESS BY INDEX ROWID| DEPARTMENTS | 1 | 16 | 1 (0)| 00:00:01 |
|* 4 | INDEX UNIQUE SCAN | DEPT_ID_PK | 1 | | 0 (0)| 00:00:01 |
——————————————————————————————–

Executing DBMS_STATS to gather statistics on both the virtual and standard index. I have run tests with statistics and without and it does seem to affect virtual index access paths.

08:21:57 orcl> exec dbms_stats.gather_index_stats(‘HR’, ‘EMP2_EMP_ID_NON_VIRTUAL’);
PL/SQL procedure successfully completed.

08:23:10 orcl> exec dbms_stats.gather_index_stats(‘HR’, ‘EMP2_EMP_ID_VIRTUAL’);

PL/SQL procedure successfully completed.

Looking for information on indexes built on the EMPLOYEES2 table.
Oracle returns a row for the standard index but not the virtual index.

08:20:31 orcl> select index_name, last_analyzed from dba_indexes where table_name = ‘EMPLOYEES2’

INDEX_NAME LAST_ANAL
—————————— ———
EMP2_EMP_ID_NON_VIRTUAL 31-MAY-07

Determining f we can find the virtual index in DBA_SEGMENTS. No success.

08:26:09 orcl> select segment_name, segment_type from dba_segments where segment_name like ‘EMP2%’;
SEGMENT_NAME SEGMENT_TYPE
——————– ——————
EMP2_EMP_ID_NON_VIRT INDEX
UAL

Looking for the the virtual index in DBA_OBJECTS. Finally, we find some sort of evidence that the virtual index exists in the database!

08:30:21 orcl> col object_name for a30
08:30:29 orcl> r
1 select object_name, object_type, created, status, temporary
2* from dba_objects where object_name like ‘EMP2%’

OBJECT_NAME OBJECT_TYPE CREATED STATUS T
—————————— ——————- ——— ——- –
EMP2_EMP_ID_NON_VIRTUAL INDEX 31-MAY-07 VALID N
EMP2_EMP_ID_VIRTUAL INDEX 31-MAY-07 VALID N