Oracle 11g 新特性——隐式索引（Invisible Index）
转载自：http://space.itpub.net/23161469/viewspace-751274
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)