从9.2版本开始Oracle引入了虚拟索引的概念，虚拟索引是一个“伪造”的索引，它的定义只存在数据字典中并有存在相关的索引段。虚拟索引是为了在不真正创建索引的情况下，验证如果使用索引sql执行计划是否改变，执行效率是否能得到提高。
本文在11.2.0.4版本中测试使用虚拟索引
1、创建测试表
ZX@orcl> create table test_t as select * from dba_objects;
Table created.
ZX@orcl> select count(*) from test_t;
COUNT(*)
----------
86369
2、查看一个SQL的执行计划，由于没有创建索引，使用TABLE ACCESS FULL访问表
ZX@orcl> set autotrace traceonly explain
ZX@orcl> select object_name from test_t where object_id=123;
Execution Plan
----------------------------------------------------------
Plan hash value: 2946757696
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1106 | 344 (1)| 00:00:05 |
|* 1 | TABLE ACCESS FULL| TEST_T | 14 | 1106 | 344 (1)| 00:00:05 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID"=123)
Note
-----
- dynamic sampling used for this statement (level=2)
3、创建虚拟索引，数据字典中有这个索引的定义但是并没有实际创建这个索引段
ZX@orcl> set autotrace off
ZX@orcl> create index idx_virtual on test_t (object_id) nosegment;
Index created.
ZX@orcl> select object_name,object_type from user_objects where object_name='IDX_VIRTUAL';
OBJECT_NAME OBJECT_TYPE
-------------------------------------------------------------------------------------------------------------------------------- -------------------
IDX_VIRTUAL INDEX
ZX@orcl> select segment_name,tablespace_name from user_segments where segment_name='IDX_VIRTUAL';
no rows selected
4、再次查看执行计划
ZX@orcl> set autotrace traceonly explain
ZX@orcl> select object_name from test_t where object_id=123;
Execution Plan
----------------------------------------------------------
Plan hash value: 2946757696
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1106 | 344 (1)| 00:00:05 |
|* 1 | TABLE ACCESS FULL| TEST_T | 14 | 1106 | 344 (1)| 00:00:05 |
----------------------------------------------------------------------------
5、我们看到执行计划并没有使用上面创建的索引，要使用虚拟索引需要设置参数
ZX@orcl> alter session set "_use_nosegment_indexes"=true;
Session altered.
6、再次查看执行计划，可以看到执行计划选择了虚拟索引，而且时间也缩短了。
ZX@orcl> select object_name from test_t where object_id=123;
Execution Plan
----------------------------------------------------------
Plan hash value: 1533029720
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1106 | 5 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST_T | 14 | 1106 | 5 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_VIRTUAL | 315 | | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"=123)
Note
-----
- dynamic sampling used for this statement (level=2)
从上面的执行计划可以看出创建这个索引会起到优化的效果，这个功能在大表建联合索引优化能起到很好的做作用，可以测试多个列组合哪个组合效果最好，而不需要实际每个组合都创建一个大索引。
7、删除虚拟索引
ZX@orcl> drop index idx_virtual;
Index dropped.
MOS文档：Virtual Indexes (文档 ID 1401046.1)
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