Invisible Index --Oracle 11g 新特性

Oracle 11g 开始，可以创建隐式索引。优化器会忽略隐式索引，除非在系统或者会话级别设置初始化参数OPTIMIZER_USE_INVISIBLE_INDEXES = TRUE，该参数默认是FALSE。隐式索引用来替代使索引不可用或者删除索引。通过隐式索引，可以完成如下操作：

（1）
在删除索引之前测试要删除的索引。

（2）
对运用程序特定的操作或模块使用临时索引结构，从而不影响整个运用。

与不可用索引不同，隐式索引在执行DML期间仍然会维护。

创建隐式索引：

CREATE INDEX index_name ON TABLE_NAME ( column_name ) INVISIBLE;

或者通过使用ALTER INDEX命令修改：

ALTER INDEX index_name INVISIBLE;

ALTER INDEX index_name VISIBLE;

通过*_INDEXES数据字典视图的VISIBILITY列来确定索引是VISIBLE还是INVISIBLE。

示例：

----创建测试表：

SQL> create table ii_test (

2 i number )

3 ;

Table created.

----插入测试数据：

SQL> begin

2 for i in 1.. 10000 loop

3 insert into ii_test values (i);

4 end loop;

5 commit;

6 end;

7 /

PL/SQL procedure successfully completed.

----创建隐式索引：

SQL> create index ii_test_id on ii_test(i) invisible;

Index created.

----收集统计信息：

SQL>EXEC DBMS_STATS.GATHER_TABLE_STATS(ownname=>'test',tabname=>'ii_test',cascade=>true);

PL/SQL procedure successfully completed.

----where条件查询：

SQL> set autot trace exp

SQL> select * from ii_test where i=580;

Execution Plan

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

Plan hash value:
4113430834

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

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

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

| 0 | SELECT STATEMENT | | 1 | 4 | 7 (0)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| II_TEST | 1 | 4 | 7 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - filter("I"=580)

----在会话级修改OPTIMIZER_USE_INVISIBLE_INDEXES = TRUE并在次测试：

SQL> alter session set OPTIMIZER_USE_INVISIBLE_INDEXES = TRUE;

Session altered.

SQL> select * from ii_test where i=580;

Execution Plan

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

Plan hash value: 3143506905

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

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

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

| 0 | SELECT STATEMENT | | 1 | 4 | 1 (0)| 00:00:01 |

|* 1 | INDEX RANGE SCAN| II_TEST_ID | 1 | 4 | 1 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - access("I"=580)

----设置OPTIMIZER_USE_INVISIBLE_INDEXES = FALSE，并使用hint测试：

SQL> alter session set OPTIMIZER_USE_INVISIBLE_INDEXES = FALSE ;

Session altered.

SQL> select /*+index(test
ii_test_id)*/ * from ii_test where i=580;

Execution Plan

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

Plan hash value: 4113430834

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

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

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

| 0 | SELECT STATEMENT | | 1 | 4 | 7 (0)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| II_TEST | 1 | 4 | 7 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - filter("I"=580)

由此可见对于invisible的索引，hint无效；

----将索引改为visible测试：

SQL> alter index ii_test_id visible;

Index altered.

SQL> select * from ii_test where i=580;

Execution Plan

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

Plan hash value: 3143506905

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

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

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

| 0 | SELECT STATEMENT | | 1 | 4 | 1 (0)| 00:00:01 |

|* 1 | INDEX RANGE SCAN| II_TEST_ID | 1 | 4 | 1 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - access("I"=580)