Oracle虚拟索引

2017-01-12 09:53:43来源:作者:Linux公社人点击

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)

更多Oracle相关信息见 Oracle 专题页面 http://www.linuxidc.com/topicnews.aspx?tid=12

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