为什么 /*+ NO_INDEX*/ 不提示，影响“执行计划”？

我有一个具有以下结构的示例表：

create table mi_dimcustomer
(customer_num    number(10),
 <other columns> <data types>)

并且有一个unique indexon 列customer_num。我试图hint the optimizer不像这样使用这个索引（只是为了练习）：

select /*+gather_plan_statistics*/ /*+ no_index(t idx1_dimcustomer) */
 *
  from mi_dimcustomer t
 where t.customer_num = 12;

但是在执行计划中，我可以看到优化器还在使用索引！这是我捕获执行计划的方式：

Step-1)

    select sql_id, child_number, sql_text
      from v$sql
     where sql_text like '%where t.CUSTOMER_NUM = 12%';

Step-2)    

select *
  from table(dbms_xplan.display_cursor('2qataxp9mahpj',
                                       '0',
                                       'ALLSTATS LAST +COST +OUTLINE'))

您可以在下面看到执行计划：

SQL_ID  2qataxp9mahpj, child number 0
-------------------------------------
select /*+gather_plan_statistics*//*+ NO_INDEX(t idx1_dimcustomer) */ * 
from mi_dimcustomer t where t.CUSTOMER_NUM = 12
 
Plan hash value: 3784660444
 
-----------------------------------------------------------------------------------------------------------------------
| Id  | Operation                   | Name             | Starts | E-Rows | Cost (%CPU)| A-Rows |   A-Time   | Buffers |
-----------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                  |      1 |        |     2 (100)|      1 |00:00:00.01 |       3 |
|   1 |  TABLE ACCESS BY INDEX ROWID| MI_DIMCUSTOMER   |      1 |      1 |     2   (0)|      1 |00:00:00.01 |       3 |
|*  2 |   INDEX UNIQUE SCAN         | IDX1_DIMCUSTOMER |      1 |      1 |     1   (0)|      1 |00:00:00.01 |       2 |
-----------------------------------------------------------------------------------------------------------------------

为什么会这样？

提前致谢