在数据库优化中，索引的重要性不言而喻。但是，在性能调整过程中，一个索引是否能被查询用到，在索引创建之前是无法确定的，而创建索引是一个代价比较高的操作，尤其是数据量较大的时候。

虚拟索引不是物理存在的，它并不会创建实际的索引段，只是在数据字典中加了一个索引的记录，使得优化器能够意识到一个索引的存在，从而判断是否使用该索引作为访问路径。当然，实际上最终查询的访问路径是不会使用该虚拟索引的。

所以，虚拟索引的用处就是用来判断一个索引对于sql的执行计划的影响，尤其是对整个数据库的影响，从而判断是否需要创建物理索引。

oracle文档中并没有提到虚拟索引的创建语法，实际上就是普通索引语法后面加一个nosegment关键字即可，B*Tree index和bitmap index都可以。

不同版本的虚拟索引的特性可能不一样，本文的例子执行环境为：

代码:

NING@ning>select * from v$version;


BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Prod

PL/SQL Release 10.2.0.3.0 - Production

CORE    10.2.0.3.0      Production

TNS for 32-bit Windows: Version 10.2.0.3.0 - Production

NLSRTL Version 10.2.0.3.0 - Production

1.创建虚拟索引

代码:

NING@ning>create table test(id int,name varchar2(30));


Table created.


NING@ning>insert into test select rownum,object_name from all_objects where rownum<1001;


1000 rows created.


NING@ning>commit;


Commit complete.


NING@ning>create unique index ix_test on test(id) nosegment;


Index created.


NING@ning>analyze table test compute statistics;


Table analyzed.

2.使用虚拟索引

代码:

NING@ning>explain plan for select * from test where id=1;


Explained.


NING@ning>select * from table(dbms_xplan.display());


PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1357081020


--------------------------------------------------------------------------

| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |

--------------------------------------------------------------------------

|   0 | SELECT STATEMENT  |      |     1 |    17 |     3   (0)| 00:00:01 |

|*  1 |  TABLE ACCESS FULL| TEST |     1 |    17 |     3   (0)| 00:00:01 |

--------------------------------------------------------------------------

必须设置隐含参数"_use_nosegment_indexes"=true（默认为false)后，CBO才能使用虚拟索引ix_test

代码:

NING@ning>alter session set "_use_nosegment_indexes"=true;


Session altered.


NING@ning>select * from table(dbms_xplan.display());


PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------
Plan hash value: 166686173


---------------------------------------------------------------------------------------

| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |

---------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT            |         |     1 |    17 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| TEST    |     1 |    17 |     2   (0)| 00:00:01 |

|*  2 |   INDEX UNIQUE SCAN         | IX_TEST |     1 |       |     1   (0)| 00:00:01 |

---------------------------------------------------------------------------------------

RBO无法使用虚拟索引

代码:

NING@ning>alter session set optimizer_mode=rule;


Session altered.


NING@ning>explain plan for select * from test where id=1;


Explained.


NING@ning>select * from table(dbms_xplan.display());


PLAN_TABLE_OUTPUT
-----------------------------------------------------------
Plan hash value: 1357081020


----------------------------------

| Id  | Operation         | Name |

----------------------------------

|   0 | SELECT STATEMENT  |      |

|*  1 |  TABLE ACCESS FULL| TEST |

----------------------------------

RBO使用hint可以使用虚拟索引

代码:

NING@ning>explain plan for select /*+ index(test,ix_test)*/* from test where id=1;


Explained.


NING@ning>select * from table(dbms_xplan.display());


PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------
Plan hash value: 166686173


---------------------------------------------------------------------------------------

| Id  | Operation                   | Name    | Rows  | Bytes | Cost (%CPU)| Time     |

---------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT            |         |     1 |    17 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| TEST    |     1 |    17 |     2   (0)| 00:00:01 |

|*  2 |   INDEX UNIQUE SCAN         | IX_TEST |     1 |       |     1   (0)| 00:00:01 |

---------------------------------------------------------------------------------------

3.虚拟索引的特性
无法执行alter index

代码:

NING@ning>alter index ix_test rebuild;
alter index ix_test rebuild
*
ERROR at line 1:
ORA-08114: can not alter a fake index



NING@ning>alter index ix_test rename to ix_test2;
alter index ix_test rename to ix_test2
*
ERROR at line 1:
ORA-08114: can not alter a fake index

不能创建和虚拟索引同名的实际索引

代码:

NING@ning>create index ix_test on test(name);
create index ix_test on test(name)

             *
ERROR at line 1:
ORA-00955: name is already used by an existing object

可以创建和虚拟索引包含相同列但不同名的实际索引

代码:

NING@ning>create index ix_test2 on test(id);


Index created.

在10g使用回收站特性的时候，虚拟索引必须显式drop，或者在drop table后purge table后，才能创建同名的索引

代码:

NING@ning>drop table test;


Table dropped.


NING@ning>create unique index ix_test on test2(id);
create unique index ix_test on test2(id)

                    *
ERROR at line 1:
ORA-00955: name is already used by an existing object



NING@ning>drop index ix_test;
drop index ix_test

           *
ERROR at line 1:
ORA-38301: can not perform DDL/DML over objects in Recycle Bin



NING@ning>purge table test;


Table purged.


NING@ning>create unique index ix_test on test2(id);


Index created.