alter system set parallel_max_servers=16;
alter system set parallel_degree_limit=8;
alter system set parallel_min_time_threshold=1 scope=memory;
drop table t purge;

create table t noparallel as select dummy, rpad(level,400) padding from dual connect by level<=1000;
call dbms_stats.gather_table_stats('','t');
call dbms_stats.set_table_stats(user,'T',numrows => 1e7, numblks => 1e6);

alter table t parallel 4;
select t.degree from user_tables t where table_name='T';

alter table t noparallel;
select t.degree from user_tables t where table_name='T';

pause;

alter session set parallel_degree_policy=auto;

explain plan for update t set dummy='Y';
select * from table(dbms_xplan.display);
pause;

SQL> alter system set parallel_max_servers=16;
SQL> alter system set parallel_degree_limit=8;
SQL> alter system set parallel_min_time_threshold=1 scope=memory;
SQL> alter table t noparallel;

Table altered.

SQL> select t.degree from user_tables t where table_name='T';

DEGREE
----------------------------------------
         1

1 row selected.

SQL> alter session set parallel_degree_policy=auto;

Session altered.

SQL> explain plan for update t set dummy='Y';

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------
Plan hash value: 1378397380

---------------------------------------------------------------------------------------------------------------
| Id  | Operation             | Name     | Rows  | Bytes | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
---------------------------------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT      |          |    10M|    19M|   100K  (1)| 00:00:04 |        |      |            |
|   1 |  UPDATE               | T        |       |       |            |          |        |      |            |
|   2 |   PX COORDINATOR      |          |       |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM)| :TQ10000 |    10M|    19M|   100K  (1)| 00:00:04 |  Q1,00 | P->S | QC (RAND)  |
|   4 |     PX BLOCK ITERATOR |          |    10M|    19M|   100K  (1)| 00:00:04 |  Q1,00 | PCWC |            |
|   5 |      TABLE ACCESS FULL| T        |    10M|    19M|   100K  (1)| 00:00:04 |  Q1,00 | PCWP |            |
---------------------------------------------------------------------------------------------------------------

Note
-----
   - automatic DOP: Computed Degree of Parallelism is 3