Re: HowTo find out used (useful) and unused (usesless) indexes?

From: DA Morgan <damorgan_at_psoug.org>
Date: Thu, 17 Apr 2008 10:02:24 -0700
Message-ID: <1208451744.272589@bubbleator.drizzle.com>

fitzjarrell_at_cox.net wrote:
> On Apr 17, 4:38 am, Andreas Mosmann <mosm..._at_expires-30-04-2008.news-
> group.org> wrote:

>> Hi,
>>
>> Is there a way to find out what indexes are used(useful)/unused(useless)
>> while the last few days/weeks?
>> If possible so tell me a way via data dictionary without using any
>> external tools.
>>
>> Andreas Mosmann
>>
>> --
>> wenn email, dann AndreasMosmann <bei> web <punkt> de

>
> Let Oracle do that for you:
>
> SQL> create index emp_eno_idx
> 2 on emp(empno);
>
> Index created.
>
> SQL>
> SQL> alter index emp_eno_idx monitoring usage;
>
> Index altered.
>
> SQL>
> SQL> select * from emp;
>
> EMPNO ENAME JOB MGR HIREDATE SAL
> COMM DEPTNO
> ---------- ---------- --------- ---------- --------- ----------
> ---------- ----------
> 7369 SMITH CLERK 7902 17-DEC-80
> 800 20
> 7499 ALLEN SALESMAN 7698 20-FEB-81 1600
> 300 30
> 7521 WARD SALESMAN 7698 22-FEB-81 1250
> 500 30
> 7566 JONES MANAGER 7839 02-APR-81
> 2975 20
> 7654 MARTIN SALESMAN 7698 28-SEP-81 1250
> 1400 30
> 7698 BLAKE MANAGER 7839 01-MAY-81
> 2850 30
> 7782 CLARK MANAGER 7839 09-JUN-81
> 2450 10
> 7788 SCOTT ANALYST 7566 09-DEC-82
> 3000 20
> 7839 KING PRESIDENT 17-NOV-81
> 5000 10
> 7844 TURNER SALESMAN 7698 08-SEP-81
> 1500 0 30
> 7876 ADAMS CLERK 7788 12-JAN-83
> 1100 20
>
> EMPNO ENAME JOB MGR HIREDATE SAL
> COMM DEPTNO
> ---------- ---------- --------- ---------- --------- ----------
> ---------- ----------
> 7900 JAMES CLERK 7698 03-DEC-81
> 950 30
> 7902 FORD ANALYST 7566 03-DEC-81
> 3000 20
> 7934 MILLER CLERK 7782 23-JAN-82
> 1300 10
>
> 14 rows selected.
>
> SQL>
> SQL> select index_name, table_name, used from v$object_usage;
>
> INDEX_NAME TABLE_NAME USE
> ------------------------------ ------------------------------ ---
> EMP_ENO_IDX EMP NO
>
> SQL>
> SQL> select * from emp where deptno = 30;
>
> EMPNO ENAME JOB MGR HIREDATE SAL
> COMM DEPTNO
> ---------- ---------- --------- ---------- --------- ----------
> ---------- ----------
> 7499 ALLEN SALESMAN 7698 20-FEB-81 1600
> 300 30
> 7521 WARD SALESMAN 7698 22-FEB-81 1250
> 500 30
> 7654 MARTIN SALESMAN 7698 28-SEP-81 1250
> 1400 30
> 7698 BLAKE MANAGER 7839 01-MAY-81
> 2850 30
> 7844 TURNER SALESMAN 7698 08-SEP-81
> 1500 0 30
> 7900 JAMES CLERK 7698 03-DEC-81
> 950 30
>
> 6 rows selected.
>
> SQL>
> SQL> select index_name, table_name, used from v$object_usage;
>
> INDEX_NAME TABLE_NAME USE
> ------------------------------ ------------------------------ ---
> EMP_ENO_IDX EMP NO
>
> SQL>
> SQL> select * from emp where empno < 7400;
>
> EMPNO ENAME JOB MGR HIREDATE SAL
> COMM DEPTNO
> ---------- ---------- --------- ---------- --------- ----------
> ---------- ----------
> 7369 SMITH CLERK 7902 17-DEC-80
> 800 20
>
> SQL>
> SQL> select index_name, table_name, used from v$object_usage;
>
> INDEX_NAME TABLE_NAME USE
> ------------------------------ ------------------------------ ---
> EMP_ENO_IDX EMP YES
>
> SQL>
>
> Using the 'alter index <indexname> monitoring usage;' directive
> Oracle will keep track of whether or not an index is actually used and
> will do so until you issue an 'alter index <indexname> nomonitoring
> usage;' command. A simple query of V$OBJECT_USAGE will reveal which
> indexes are and are not used. After an 'alter index <indexname>
> nomonitoring usage;' directive is issued the collected data remains in
> the V$OBJECT_USAGE view; it isn't removed for the objects in question
> until another 'alter index ... monitoring usage;' command is called:
>
> SQL> alter index emp_eno_idx nomonitoring usage;
>
> Index altered.
>
> SQL> select index_name, table_name,used from v$object_usage;
>
> INDEX_NAME TABLE_NAME USE
> ------------------------------ ------------------------------ ---
> EMP_ENO_IDX EMP YES
>
> SQL>
>
>
> David Fitzjarrell

One small addendum to David's excellent response.

Sometimes Oracle uses the statistics collected for an index but does not use the index itself.

Thus it is possible to drop an unused index and have performance affected. If you see this happen then rebuild the index and note what is happening for future reference.

-- 
Daniel A. Morgan
Oracle Ace Director & Instructor
University of Washington
damorgan_at_x.washington.edu (replace x with u to respond)
Puget Sound Oracle Users Group
www.psoug.org

Received on Thu Apr 17 2008 - 12:02:24 CDT