Why isn't index used for this query?

Question

I had a query where an index was not used when I thought it could be, so I reproduced it out of curiosity:

Create a test_table with 1.000.000 rows (10 disti

Answer 1

bitmap index will do as well

Execution Plan
----------------------------------------------------------
Plan hash value: 2200191467

---------------------------------------------------------------------------------
| Id  | Operation          | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |            |    10 |    30 | 15983   (2)| 00:03:12 |
|   1 |  HASH GROUP BY     |            |    10 |    30 | 15983   (2)| 00:03:12 |
|   2 |   TABLE ACCESS FULL| TEST_TABLE |  1013K|  2968K| 15825   (1)| 00:03:10 |
---------------------------------------------------------------------------------

SQL> create bitmap index test_index on test_table(col);

Index created.

SQL> EXEC dbms_stats.gather_table_stats( 'MY_SCHEMA', 'TEST_TABLE' );

PL/SQL procedure successfully completed.

SQL> SELECT col, COUNT(*)
  2    FROM test_table
  3    GROUP BY col
  4  /

Execution Plan
----------------------------------------------------------
Plan hash value: 238193838

---------------------------------------------------------------------------------------
| Id  | Operation                | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |            |    10 |    30 |   286   (0)| 00:00:04 |
|   1 |  SORT GROUP BY NOSORT    |            |    10 |    30 |   286   (0)| 00:00:04 |
|   2 |   BITMAP CONVERSION COUNT|            |  1010K|  2961K|   286   (0)| 00:00:04 |
|   3 |    BITMAP INDEX FULL SCAN| TEST_INDEX |       |       |            |          |
---------------------------------------------------------------------------------------