Oracle中怎么实现虚拟索引

Oracle中怎么实现虚拟索引，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。

1.创建一个测试表test

SQL> create table test as select * from dba_objects; Table created.

2.从表test查询object_name等于standard的记录

SQL> select * from test where object_name='STANDARD';  OWNER  ------------------------------  OBJECT_NAME  --------------------------------------------------------------------  SUBOBJECT_NAME OBJECT_ID DATA_OBJECT_ID OBJECT_TYPE  ------------------------------ ---------- -------------- -----------  CREATED LAST_DDL_TIM TIMESTAMP STATUS T G S  ------------ ------------ ------------------- ------- - - -  SYS  STANDARD  888 PACKAGE  19-APR-10 19-APR-10 2003-04-18:00:00:00 VALID N N N  OWNER  ------------------------------  OBJECT_NAME  --------------------------------------------------------------------  SUBOBJECT_NAME OBJECT_ID DATA_OBJECT_ID OBJECT_TYPE  ------------------------------ ---------- -------------- -------------------  CREATED LAST_DDL_TIM TIMESTAMP STATUS T G S  ------------ ------------ ------------------- ------- - - -  SYS  STANDARD  889 PACKAGE BODY  19-APR-10 19-APR-10 2010-04-19:10:22:58 VALID N N N

3.查询上面查询的执行计划

SQL> set autotrace traceonly explain  SQL> select * from test where object_name='STANDARD';  Execution Plan  ----------------------------------------------------------  Plan hash value: 1357081020  --------------------------------------------------------------------------  | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |  --------------------------------------------------------------------------  | 0 | SELECT STATEMENT | | 8 | 1416 | 155 (1)| 00:00:02 |  |* 1 | TABLE ACCESS FULL| TEST | 8 | 1416 | 155 (1)| 00:00:02 |  --------------------------------------------------------------------------  Predicate Information (identified by operation id):  ---------------------------------------------------  1 - filter("OBJECT_NAME"='STANDARD')  Note  -----  - dynamic sampling used for this statement

4.在表test的object_name列上创建一个虚拟索引

SQL> create index test_index on test(object_name) nosegment;Index created.

为了创建虚拟索引必须在create index语句中指定nosegment子句，并且不会创建索引段。

5.来验证虚拟索引不会创建索引段

SQL> set autotrace off  SQL> select index_name from dba_indexes where table_name = 'TEST' and index_name = 'TEST_INDEX';  no rows selected  SQL> col OBJECT_NAME format a20;  SQL> select object_name, object_type from dba_objects where object_name = 'TEST_INDEX';  OBJECT_NAME OBJECT_TYPE  -------------------- -------------------  TEST_INDEX INDEX

从上面的结果可以看到索引对象已经创建，但没有创建索引段。

6.重新执行sql查看创建的虚拟索引是否被使用

SQL> set autotrace traceonly explainSQL> select * from test where object_name='STANDARD';  Execution Plan  ----------------------------------------------------------  Plan hash value: 1357081020  --------------------------------------------------------------------  | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |  --------------------------------------------------------------------  | 0 | SELECT STATEMENT | | 8 | 1416 | 155 (1)| 00:00:02 |  |* 1 | TABLE ACCESS FULL| TEST | 8 | 1416 | 155 (1)| 00:00:02 |  --------------------------------------------------------------------  Predicate Information (identified by operation id):  ---------------------------------------------------  1 - filter("OBJECT_NAME"='STANDARD')  Note  -----  - dynamic sampling used for this statement

从上面的执行计划可以清楚地看到创建的虚拟索引并没有被使用。

7.为了能使用所创建的虚拟索引，需要将_USE_NOSEGMENT_INDEXES设置为true

SQL> alter session set "_USE_NOSEGMENT_INDEXES" = true;Session altered.

8.重新执行sql查看创建的虚拟索引是否被使用

SQL> set long 900SQL> set linesize 900  SQL> select * from test where object_name='STANDARD';  Execution Plan  ----------------------------------------------------------  Plan hash value: 2627321457  --------------------------------------------------------------------  | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |  --------------------------------------------------------------------  | 0 | SELECT STATEMENT | | 8 | 1416 | 5 (0)| 00:00:01 |  | 1 | TABLE ACCESS BY INDEX ROWID| TEST | 8 | 1416 | 5 (0)| 00:00:01 |  |* 2 | INDEX RANGE SCAN | TEST_INDEX | 238 | | 1 (0)| 00:00:01 |  --------------------------------------------------------------------  Predicate Information (identified by operation id):  ---------------------------------------------------  2 - access("OBJECT_NAME"='STANDARD')  Note  -----  - dynamic sampling used for this statement

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