Oracle 监控索引特性为我们提供了一个大致判断索引是否被使用的情形。之所以这么说，是因为在Oracle 10g 中收集统计信息时会导致索引被监控，此并非sql语句而产生。而在11g则不会出现类型的情形。其次对于存在子表存在外键的情形，对于主表进行操作时是否会导致索引被监控呢？下面描述的是这个话题。
1、普通监控索引的情形
--演示环境
SQL> select * from v$version where rownum create table ptb(deptno number constraint ptb_pk primary key,dname varchar2(20));
Table created.
--从scott.dept帐户复制数据
SQL> insert into ptb select deptno,dname from dept;
4 rows created.
SQL> commit;
Commit complete.
--开启索引监控
SQL> alter index ptb_pk monitoring usage;
--为主表收集统计信息
SQL> exec dbms_stats.gather_table_stats('SCOTT','PTB',cascade=>true);
PL/SQL procedure successfully completed
SQL> select * from v$object_usage where index_name='PTB_PK';
INDEX_NAME TABLE_NAME MON Use START_MONITORING END_MONITORING
------------------------------ ------------------------- --- --- ------------------- -------------------
PTB_PK PTB YES NO 03/22/2013 17:15:37
--注意上面的情形，收集统计信息时，索引被使用没有被监控到，在10g中则会被监控到
--下面开启autotrace
SQL> set autot trace exp;
SQL> select * from ptb where deptno=10;
Execution Plan
----------------------------------------------------------
Plan hash value: 3991869509
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12 | 1 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| PTB | 1 | 12 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PTB_PK | 1 | | 0 (0)| 00:00:01 |
--------------------------------------------------------------------------------------
SQL> set autot off;
SQL> select * from v$object_usage where index_name='PTB_PK'; --索引使用被监控到
INDEX_NAME TABLE_NAME MON Use START_MONITORING END_MONITORING
------------------------------ ------------------------- --- --- ------------------- -------------------
PTB_PK PTB YES YES 03/22/2013 17:15:37