How to force the optimaizer to stop using an index ?

SQL>  create table t
  2   as
  3   select level no, rpad('*',100,'*') name
  4     from dual
  5  connect by level <= 100000
  6  /

Table created.

SQL> create index i on t(no)
  2  /

Index created.

SQL> exec dbms_stats.gather_table_stats(user,'T',cascade=>true)

PL/SQL procedure successfully completed.

SQL> set autotrace traceonly explain

SQL> select * from t where no = 25000
  2  /

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=2 Card=1 Bytes=105)
   1    0   TABLE ACCESS (BY INDEX ROWID) OF 'T' (TABLE) (Cost=2 Card=1 Bytes=105)
   2    1     INDEX (RANGE SCAN) OF 'I' (INDEX) (Cost=1 Card=1)



SQL> select /*+ NO_INDEX(t i) */ * from t where no = 25000
  2  /

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=350 Card=1 Bytes=105)
   1    0   TABLE ACCESS (FULL) OF 'T' (TABLE) (Cost=350 Card=1 Bytes=105)

SQL> drop table t purge;

Table dropped.

SQL> create table t ( a number);

Table created.

SQL> create index t_idx on t(a);

Index created.

SQL>
SQL> insert into t select level from dual connect by level <=100;

100 rows created.

SQL> commit;

Commit complete.

SQL>
SQL> exec dbms_stats.gather_table_stats(user, 't', cascade => true, method_opt => 'for all columns size 1');

PL/SQL procedure successfully completed.

SQL>
SQL> set autotrace traceonly exp
SQL>
SQL> select * from t where a = 1;

Execution Plan
----------------------------------------------------------
Plan hash value: 2946670127

--------------------------------------------------------------------------
| Id  | Operation        | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT |       |     1 |     3 |     1   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| T_IDX |     1 |     3 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("A"=1)

SQL>
SQL> alter table t move;

Table altered.

SQL>
SQL> select * from t where a = 1;

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |     3 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |     1 |     3 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("A"=1)

SQL>
SQL> set autotrace off
SQL> select status from user_indexes where index_name = 'T_IDX';

STATUS
--------
UNUSABLE

SQL> alter index t_idx rebuild;

Index altered.

SQL>