对于索引的调整，我们可以通过Oracle提供的索引监控特性来跟踪索引是否被使用。尽管该特性并未提供索引使用的频度，但仍不失为我们参考的方式之一。然而，最近在Oracle 10.2.0.3中发现收集统计信息时导致索引也被监控，而不是用于sql查询引发的索引监控。如此这般，索引监控岂不是鸡肋？
1、基于Oracle 10g 收集统计信息索引被监控情形
scott@CNMMBO> select * from v$version where rownum<2;
BANNER
----------------------------------------------------------------
Oracle Database 10g Release 10.2.0.3.0 - 64bit Production
--创建临时表t
scott@CNMMBO> create table t(id number constraint t_pk primary key);
Table created.
--启用索引监控
scott@CNMMBO> alter index t_pk monitoring usage;
Index altered.
--查看对象的使用情况
scott@CNMMBO> select * from v$object_usage where index_name='T_PK';
INDEX_NAME Table Name MON USE START_MONITORING END_MONITORING
------------------------------ ----------------- --- --- ------------------- -------------------
T_PK T YES NO 03/22/2013 20:53:23
--收集表t上的统计信息
scott@CNMMBO> exec dbms_stats.gather_table_stats('SCOTT','T',cascade=>true);
PL/SQL procedure successfully completed.
--下面的查询中提示索引没有被使用
--这应该是由于表上没有数据的缘故，也就不存在对应的索引段
scott@CNMMBO> select * from v$object_usage where index_name='T_PK';
INDEX_NAME Table Name MON USE START_MONITORING END_MONITORING
------------------------------ ----------------- --- --- ------------------- -------------------
T_PK T YES NO 03/22/2013 20:53:23
--下面尝试插入两条数据
scott@CNMMBO> insert into t select 1 from dual;
1 row created.
scott@CNMMBO> insert into t select 2 from dual;
1 row created.
--再次收集统计信息
scott@CNMMBO> exec dbms_stats.gather_table_stats('SCOTT','T',cascade=>true);
PL/SQL procedure successfully completed.
--Author : Robinson
--Blog : http://blog.csdn.net/robinson_0612
--这下子，索引变成了已经被使用
scott@CNMMBO> select * from v$object_usage where index_name='T_PK';
INDEX_NAME Table Name MON USE START_MONITORING END_MONITORING
------------------------------ ------------------ --- --- ------------------- -------------------
T_PK T YES YES 03/22/2013 20:53:23
2、基于Oracle 11g收集统计信息索引被监控情形
SQL> select * from v$version where rownum<2;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
--下面的操作与上面雷同
SQL> create table t(id number constraint t_pk primary key);
Table created.
SQL> alter index t_pk monitoring usage;
Index altered.
SQL> set linesize 190
SQL> select * from v$object_usage where index_name='T_PK';
INDEX_NAME TABLE_NAME MON USE START_MONITORING END_MONITORING
------------------------------ ----------------- --- --- ------------------- -------------------
T_PK T YES NO 03/22/2013 20:37:57
SQL> exec dbms_stats.gather_table_stats('SCOTT','T',cascade=>true);
PL/SQL procedure successfully completed.
SQL> select * from v$object_usage where index_name='T_PK';
INDEX_NAME TABLE_NAME MON USE START_MONITORING END_MONITORING
------------------------------ ---------------- --- --- ------------------- -------------------
T_PK T YES NO 03/22/2013 20:37:57
SQL> insert into t select 111 from dual;
1 row created.
SQL> insert into t select 222 from dual;
1 row created.
SQL> commit;
Commit complete.
SQL> exec dbms_stats.gather_table_stats('SCOTT','T',cascade=>true);
PL/SQL procedure successfully completed.
--可以看出，插入数据后，收集统计信息并不会导致索引被使用
SQL> select * from v$object_usage where index_name='T_PK';
INDEX_NAME TABLE_NAME MON USE START_MONITORING END_MONITORING
------------------------------ ---------------- --- --- ------------------- -------------------
T_PK T YES NO 03/22/2013 20:37:57
SQL> select index_name,NUM_ROWS,SAMPLE_SIZE,LAST_ANALYZED from dba_indexes where index_name='T_PK';
INDEX_NAME NUM_ROWS SAMPLE_SIZE LAST_ANALYZED
------------------------------ ---------- ----------- -------------------
T_PK 2 2 2013/03/22 20:39:10
--发布sql查询语句
SQL> select * from t where id=222;
ID
----------
222
--再次查看，索引监控提示该索引已经被使用
SQL> select * from v$object_usage where index_name='T_PK';
INDEX_NAME TABLE_NAME MON USE START_MONITORING END_MONITORING
------------------------------ ---------------- --- --- ------------------- -------------------
T_PK T YES YES 03/22/2013 20:37:57
SQL> alter index t_pk nomonitoring usage;
3、小结
a、对于索引监控，在Oracle 10g中当收集统计信息时，如果当前索引的统计信息也被收集则导致该索引被监控
b、注意索引能否被收集到还依赖于estimate_percent以及method_opt等收集时的相关参数
c、由于上述情形存在因此索引监控在10g中功能有限，不过对于索引的使用情况也可以通过查询DBA_HIST_SQL_PLAN来获得
d、在Oracle 11g中，不会出现上述情况
更多参考
PL/SQL 联合数组与嵌套表
PL/SQL 变长数组
PL/SQL --> PL/SQL记录
