SQL> create table counting as
   2   select object_id, object_name, owner
   3   from dba_objects;

Table created.

SQL> update counting set object_id = -rownum where object_id is null;

 18  rows updated.

SQL> alter table counting add constraint counting_pk
   2   primary key (object_id);

Table altered.

SQL> analyze table counting compute statistics;

Table analyzed.

SQL> analyze index counting_pk compute statistics;

Index analyzed.

SQL> set autotrace on;
SQL> select count(*) from counting;

  COUNT(*)                                                                      
----------                                                                      
      41725                                                                       


Execution Plan
----------------------------------------------------------                      
    0       SELECT STATEMENT Optimizer=CHOOSE (Cost= 10  Card= 1 )                    
    1      0    SORT (AGGREGATE)                                                    
    2      1      INDEX (FAST FULL SCAN) OF 'COUNTING_PK' (UNIQUE)