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

you forgot this really important information: COL is not null

If the column is NULLABLE, the index can not be used because there might be unindexed rows.

SQL> ALTER TABLE test_table MODIFY (col NOT NULL);

Table altered
SQL> EXPLAIN PLAN FOR
  2  SELECT col, COUNT(*) FROM test_table GROUP BY col;

Explained
SQL> SELECT * FROM table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1077170955
--------------------------------------------------------------------------------
| Id  | Operation            | Name       | Rows  | Bytes | Cost (%CPU)| Time
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |            |    10 |    30 |  1954   (1)| 00:00:2
|   1 |  SORT GROUP BY NOSORT|            |    10 |    30 |  1954   (1)| 00:00:2
|   2 |   INDEX FULL SCAN    | TEST_INDEX |   976K|  2861K|  1954   (1)| 00:00:2
--------------------------------------------------------------------------------