SQL> drop table t;

Table dropped.

SQL> create table t partition by list(owner)
  2  (Partition SYS_P Values('SYS'),Partition System_P Values('SYSTEM'),
  3  Partition Thiru_P values('THIRU'),Partition Other_P values(Default))
  4  as select * from all_objects;

Table created.

SQL> analyze table t estimate statistics;

Table analyzed.

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

  COUNT(*)
----------
     30802

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=45 Card=1)
   1    0   SORT (AGGREGATE)
   2    1     PARTITION LIST (ALL)
   3    2       TABLE ACCESS (FULL) OF 'T' (Cost=45 Card=32561)

SQL> alter table t parallel;

Table altered.

SQL> select count(*) from t;

  COUNT(*)
----------
     30802

Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=12 Card=1)
   1    0   SORT (AGGREGATE)
   2    1     SORT* (AGGREGATE)                                        :Q16000
   3    2       PARTITION LIST* (ALL)                                  :Q16000
   4    3         TABLE ACCESS* (FULL) OF 'T' (Cost=12 Card=32561)     :Q16000

   2 PARALLEL_TO_SERIAL            SELECT /*+ PIV_SSF */ SYS_OP_MSR(COUNT(*)) F
                                   ROM (SELECT /*+ NO_EXPAND ROWID(A2) */ 0 FRO
                                   M "T" PX_GRANULE(0, BLOCK_RANGE, DYNAMIC)  A
                                   2) A1

   3 PARALLEL_COMBINED_WITH_PARENT
   4 PARALLEL_COMBINED_WITH_PARENT