Oracle虚拟索引

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 explainZX@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 offZX@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                                                                                                                      INDEXZX@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 explainZX@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)