SQL> explain plan for SELECT o.owner, o.object_type, COUNT(*)
  2  FROM all_my_objects o
  3  GROUP BY CUBE (o.object_type, o.owner);

Explained.

SQL> SELECT * FROM table(dbms_xplan.display(NULL,NULL,'all'));



Plan hash value: 2535396241

---------------------------------------------------------------------------------------

| Id  | Operation            | Name           | Rows  | Bytes | Cost (%CPU)| Time     |

---------------------------------------------------------------------------------------


|   0 | SELECT STATEMENT     |                |   339K|  9272K|  1653   (2)| 00:00:20 |

|   1 |  SORT GROUP BY       |                |   339K|  9272K|  1653   (2)| 00:00:20 |

|   2 |   GENERATE CUBE      |                |   339K|  9272K|  1653   (2)| 00:00:20 |

|   3 |    SORT GROUP BY     |                |   339K|  9272K|  1653   (2)| 00:00:20 |

|   4 |     TABLE ACCESS FULL| ALL_MY_OBJECTS |   339K|  9272K|  1640   (1)| 00:00:20 |

---------------------------------------------------------------------------------------


Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   4 - SEL$1 / O@SEL$1

Column Projection Information (identified by operation id):
-----------------------------------------------------------

   1 - (#keys=3) "O"."OBJECT_TYPE"[VARCHAR2,19], "O"."OWNER"[VARCHAR2,30], SYSDEF[4], COUNT(COUNT(*))[22]
   2 - "O"."OBJECT_TYPE"[VARCHAR2,19], "O"."OWNER"[VARCHAR2,30], SYSDEF[4], COUNT(*)[8]
   3 - (#keys=2) "O"."OBJECT_TYPE"[VARCHAR2,19], "O"."OWNER"[VARCHAR2,30], COUNT(*)[8]
   4 - "O"."OWNER"[VARCHAR2,30], "O"."OBJECT_TYPE"[VARCHAR2,19]

Note
-----
   - dynamic sampling used for this statement