SQL> begin dbms_stats.gather_table_stats(user,'dropme_t',method_opt => 'for all indexed columns size skewonly'); end;
  2  /

PL/SQL procedure successfully completed.

SQL>
SQL> var x number;
SQL>
SQL> exec :x := 1;

PL/SQL procedure successfully completed.

SQL>
SQL> SET AUTOTRACE ON;
SQL>
SQL> select count(*)
  2    from dropme_t
  3   where x1 = :x
  4     and x2 = :x
  5     and x3 = :x
  6     and x4 = :x
  7     and x5 = :x
  8     and x6 = :x
  9     ;

  COUNT(*)
----------
    500000

1 row selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 3360040140

-------------------------------------------------------------------------------
| Id  | Operation          | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |          |     1 |    18 |   704   (3)| 00:00:01 |
|   1 |  SORT AGGREGATE    |          |     1 |    18 |            |          |
|*  2 |   TABLE ACCESS FULL| DROPME_T | 15625 |   274K|   704   (3)| 00:00:01 |
-------------------------------------------------------------------------------

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

   2 - filter("X1"=TO_NUMBER(:X) AND "X2"=TO_NUMBER(:X) AND
              "X3"=TO_NUMBER(:X) AND "X4"=TO_NUMBER(:X) AND "X5"=TO_NUMBER(:X) AND
              "X6"=TO_NUMBER(:X))

SQL>