Why isn't oracle using an index for distinct query ?

Question

This is the scenario

SQL> exec dbms_stats.gather_table_stats(user,'TM', cascade=>true)

PL/SQL procedure successfully completed.

SQL> SELECT SEGMENT_NAME , SEGMENT_TYPE , BYTES / 1024 / 1024 MB , BLOCKS FROM DBA_SEGMENTS WHERE SEGMENT_NAME IN ('TM', 'TM_LD_IX');

SEGMENT_NAME            SEGMENT_TYPE        MB        BLOCKS
------------------------------------------ ---------- ----------
TM                      TABLE                 296      37888
TM_LD_IX                INDEX                  46       5888

SQL> select index_name , column_name from user_ind_columns where index_name = 'TM_LD_IX';

INDEX_NAME   COLUMN_NAME
------------ ------------------------------
TM_LD_IX     LD

SQL> explain plan for select distinct LD from TM;

Explained.

SQL> @ex

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------------------

Plan hash value: 4241255022

--------------------------------------------------------------------------------------
| Id  | Operation          | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |                 |   693 |  4158 |  7920   (8)| 00:01:36 |
|   1 |  HASH UNIQUE       |                 |   693 |  4158 |  7920   (8)| 00:01:36 |
|   2 |   TABLE ACCESS FULL| TM              |  2549K|    14M|  7486   (3)| 00:01:30 |
--------------------------------------------------------------------------------------

9 rows selected.

SQL> explain plan for select /*+ index(x , TM_LD_IX) */ distinct LD from TM x;

Explained.

SQL> @ex

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------------------

Plan hash value: 4241255022

--------------------------------------------------------------------------------------
| Id  | Operation          | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |                 |   693 |  4158 |  7920   (8)| 00:01:36 |
|   1 |  HASH UNIQUE       |                 |   693 |  4158 |  7920   (8)| 00:01:36 |
|   2 |   TABLE ACCESS FULL| TM              |  2549K|    14M|  7486   (3)| 00:01:30 |
--------------------------------------------------------------------------------------

SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Prod
PL/SQL Release 10.2.0.3.0 - Production
CORE    10.2.0.3.0      Production
TNS for 32-bit Windows: Version 10.2.0.3.0 - Production
NLSRTL Version 10.2.0.3.0 - Production

As you can see oracle is not using the index on LD and chooses a full table scan instead. I can't even make him use the index with a hist.

In the simple query above I would expect an index fast full scan of TM_LD_IX. my db_file_multiblock_read_count is set to 32 so i'm expecting a cost of about 5888 / 32 = 184 (using the index I can also save the cost of a hash unique).

So, what am I missing here ?