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<2;

BANNER
--------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production

--创建主表  
SQL> 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

2、存在外键时监控索引情形

SQL> create table ctb(id number,name varchar2(30),deptno number);

Table created.

--为子表添加外键约束
SQL> alter table ctb add constraint ctb_fk foreign key(deptno) references ptb(deptno)
  2  on delete set null;

Table altered.

--为子表填充数据
SQL> begin
  2  for i in 1 .. 1000000
  3  loop
  4      insert into ctb values(i,'name_'||to_char(i),10);
  5  end loop;
  6  commit;
  7  end;
  8  /

PL/SQL procedure successfully completed.

--基于外键创建索引
SQL> create index i_ctb_fk_deptno on ctb(deptno) nologging;

Index created.

--收集子表的统计信息
SQL> exec dbms_stats.gather_table_stats('SCOTT','CTB',cascade=>true);

PL/SQL procedure successfully completed.

SQL> @dba_table_info
Enter Schema (i.e. SCOTT) : SCOTT
Enter Schema (i.e. EMP) : CTB

+----------------------------------------+
| TABLE INFORMATION                                                          |
+----------------------------------------+

Owner           Table Name      Tablespace                   Last Analyzed           # of Rows
--------------- --------------- ---------------------------- -------------------- ------------
SCOTT           CTB             GOEX_SYSTEM_TBL              22-MAR-2013 17:26:02    1,000,731

+----------------------------------------+
| CONSTRAINTS                                                                |
+----------------------------------------+