SQL> set autotrace on statistics
SQL> set timing on
<third run, to have a fair comparison, post caching>
SQL> SELECT o.owner, o.object_type, COUNT(*)
  2  FROM all_my_objects o
  3  GROUP BY o.object_type, o.owner
  4  UNION ALL
  5  SELECT NULL, o.object_type, COUNT(*)
  6  FROM all_my_objects o
  7  GROUP BY o.object_type
  8  UNION ALL
  9  SELECT o.owner, NULL, COUNT(*)
 10  FROM all_my_objects o
 11  GROUP BY o.owner
 12  UNION ALL
 13  SELECT NULL, NULL, COUNT(*)
 14  FROM all_my_objects o;

OWNER                          OBJECT_TYPE           COUNT(*)                   
------------------------------ ------------------- ----------                   
<output snipped>

327 rows selected.

Elapsed: 00:00:58.26

Statistics
----------------------------------------------------------                      
          0  recursive calls                                                    
          0  db block gets                                                      
      44008  consistent gets                                                    
      24100  physical reads                                                     
          0  redo size                                                          
       9652  bytes sent via SQL*Net to client                                   
        651  bytes received via SQL*Net from client                             
         23  SQL*Net roundtrips to/from client                                  
          0  sorts (memory)                                                     
          0  sorts (disk)                                                       
        327  rows processed