CBO为什么不走索引？

原帖是Itpub上的网友提出一个CBO为什么不走索引的问题， 该问题的演示如下:  

SQL> create table maclean1 as select * from dba_objects;

Table created.

SQL> update maclean1 set status='INVALID' where owner='MACLEAN';

2 rows updated.

SQL> commit;

Commit complete.

SQL> create index ind_maclean1 on maclean1(status);

Index created.

SQL> exec dbms_stats.gather_table_stats('SYS','MACLEAN1',cascade=>true);

PL/SQL procedure successfully completed.

SQL> explain plan for select * from maclean1 where status='INVALID';

Explained.

SQL> set linesize 140 pagesize 1400
SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------
Plan hash value: 987568083

------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          | 11320 |  1028K|    85   (0)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| MACLEAN1 | 11320 |  1028K|    85   (0)| 00:00:02 |
------------------------------------------------------------------------------

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

   1 - filter("STATUS"='INVALID')

13 rows selected.

10053 trace

Access path analysis for MACLEAN1
***************************************
SINGLE TABLE ACCESS PATH