定义:虚拟索引(virtual index) 是指没有创建对应的物理段的索引。

 

虚拟索引的目的:是在不损耗主机CPU,IO,磁盘空间去实际创建索引的情况下,来判断一个索引是否能够对SQL优化起到作用。列如我们在优化一条SQL语句的时候,通常会查看需要优化的语句的执行计划,在考虑是否需要在表的某列上建立索引时就可以用到虚拟索引。虚拟索引建立的时候因为其没有消耗主机的相关资源,因此可以在相当快的时间内建立完成。

下面我们来看一下试验:

首先建立两张测试表

create table bigtab as select rownum as id,a.* from sys.all_objects a;  

create table smalltab as select rownum as id,a.* from  a;  

create table smalltab as select rownum as id,a.* from sys.all_tables a; 

多次运行以下语句,以插入多一些测试数据:

insert into bigtab select ronum as id,a.* from sys.all_objects a;

insert into smalltab select rownum as id,a.* from sys.all_tables a;    

 

查看需要执行语句的执行计划:

 

SQL> explain plan for select count(*) from bigtab a,smalltab b where a.object_name=b.table_name;

Explained.

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3089226980

--------------------------------------------------------------------------------
| Id  | Operation           | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |          |     1 |    40 |   518   (1)| 00:00:07 |
|   1 |  SORT AGGREGATE     |          |     1 |    40 |            |          |
|*  2 |   HASH JOIN         |          | 99838 |  3899K|   518   (1)| 00:00:07 |
|   3 |    TABLE ACCESS FULL| SMALLTAB | 15311 |   299K|   172   (0)| 00:00:03 |
|   4 |    TABLE ACCESS FULL| BIGTAB   | 85284 |  1665K|   345   (1)| 00:00:05 |
--------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("A"."OBJECT_NAME"="B"."TABLE_NAME")

16 rows selected.


下面我们在两个表上创建两个虚拟索引,分别在object_name和table_name列上,看看优化器是否会使用这两个索引,以及优化器的成本会如何变化。

 

SQL> show parameter _use_nosegment
SQL> alter session set "_use_nosegment_indexes"=true;

Session altered.

SQL> show parameter _use_nosegment                   

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
_use_nosegment_indexes               boolean     TRUE
SQL> create index big_ind on bigtab(object_name) nosegment;

Index created.

SQL> create index small_ind on smalltab(table_name) nosegment;                


create index语句的NOSEGMENT选项表明这个索引是“虚拟的”-----就是没有与之相关联的实际索引段。如果优化器认为这个索引有用,参数_use_nosegment_indexes将指示数据库可以在执行计划中使用这些索引。下面我们来看看如果真的创建这些索引,它们是否值得使用:

SQL> explain plan for select count(*) from bigtab a,smalltab b where a.object_name=b.table_name;

Explained.

SQL> select * from table(dbms_xplan.display());                  

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 1254475829

------------------------------------------------------------------------------------
| Id  | Operation              | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |           |     1 |    40 |    19   (6)| 00:00:01 |
|   1 |  SORT AGGREGATE        |           |     1 |    40 |            |          |
|*  2 |   HASH JOIN            |           | 99838 |  3899K|    19   (6)| 00:00:01 |
|   3 |    INDEX FAST FULL SCAN| SMALL_IND | 15311 |   299K|     9   (0)| 00:00:01 |
|   4 |    INDEX FAST FULL SCAN| BIG_IND   | 85284 |  1665K|     9   (0)| 00:00:01 |
------------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("A"."OBJECT_NAME"="B"."TABLE_NAME")

16 rows selected.


执行计划显示,这两个索引确实被认为有用,优化器估算的成本也下降到19了。所以如果需要优化语句可以建议创建这些索引。