SELECT INDEX_NAME, MONITORING, USED FROM V$OBJECT_USAGE;

no rows selected

ALTER INDEX HR.NUI_EMP1_EMPNO_ENAME MONITORING USAGE;

SELECT INDEX_NAME, MONITORING, USED FROM V$OBJECT_USAGE;

INDEX_NAME                     MON USE
------------------------------ --- ---
NUI_EMP1_EMPNO_ENAME           YES NO

insert into emp1 values (206, 'ename');
insert into emp1 values (206, 'ename')
*
ERROR at line 1:
ORA-00001: unique constraint (HR.PK_EMP1_KEY) violated

SELECT INDEX_NAME, MONITORING, USED FROM V$OBJECT_USAGE;

INDEX_NAME                     MON USE
------------------------------ --- ---
NUI_EMP1_EMPNO_ENAME           YES NO

set autotrace traceonly explain
select sum(empno) from emp1 ;

Execution Plan
-----------------------------------------------------------------------------------------
| Id  | Operation        | Name                 | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |                      |     1 |    13 |     1   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE  |                      |     1 |    13 |            |          |
|   2 |   INDEX FULL SCAN| NUI_EMP1_EMPNO_ENAME |   107 |  1391 |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

set autot off

SELECT INDEX_NAME, MONITORING, USED FROM V$OBJECT_USAGE;

INDEX_NAME                     MON USE
------------------------------ --- ---
NUI_EMP1_EMPNO_ENAME           YES YES

ALTER INDEX HR.NUI_EMP1_EMPNO_ENAME noMONITORING USAGE;

SELECT INDEX_NAME, MONITORING, USED FROM V$OBJECT_USAGE;

INDEX_NAME                     MON USE
------------------------------ --- ---
NUI_EMP1_EMPNO_ENAME           NO  YES

ALTER INDEX HR.NUI_EMP1_EMPNO_ENAME MONITORING USAGE;

SELECT INDEX_NAME, MONITORING, USED FROM V$OBJECT_USAGE;

INDEX_NAME                     MON USE
------------------------------ --- ---
NUI_EMP1_EMPNO_ENAME           YES NO