scott@TEST>alter session disable parallel query;
Session altered.
scott@TEST>select * from emp;
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1218 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| EMP | 14 | 1218 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
......
4、11gR2的自动并行
Oracle在11gR2中引入了自动并行(Auto DOP)，自动并行的开启受参数parallel_degree_policy的控制，其默认值为MANUAL，即自动并行在默认情况下并没有开启。如果通过更改PARALLEL_DEGREE_POLICY的值而开启了自动并行，那么后面执行的SQL的执行方式是串行还是并行，以及并行执行的并行度是多少等，就都是由Oracle自动来决定了。
scott@TEST>select table_name,degree from user_tables where table_name in ('EMP','EMP_TEMP');
TABLE_NAME DEGREE
------------------------------------------------------------------------------------------ ------------------------------------------------------------
EMP 1
EMP_TEMP 1
scott@TEST>alter session set parallel_degree_policy=AUTO;
Session altered.
scott@TEST>set autotrace traceonly
scott@TEST>select * from emp;
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 1218 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| EMP | 14 | 1218 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
......
scott@TEST>select * from emp_temp;
1835008 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2661083444
--------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1835K| 66M| 1683 (1)| 00:00:21 | | | |
| 1 | PX COORDINATOR | | | | | | | | |
| 2 | PX SEND QC (RANDOM)| :TQ10000 | 1835K| 66M| 1683 (1)| 00:00:21 | Q1,00 | P->S | QC (RAND) |
| 3 | PX BLOCK ITERATOR | | 1835K| 66M| 1683 (1)| 00:00:21 | Q1,00 | PCWC | |
| 4 | TABLE ACCESS FULL| EMP_TEMP | 1835K| 66M| 1683 (1)| 00:00:21 | Q1,00 | PCWP | |
--------------------------------------------------------------------------------------------------------------
......
从上面的输出可以看出表EMP和EMP_TEMP的并行度都为1，但是两个表的数据量相关很大，EMP只有14条数据，EMP_TEMP有1835008条数据。在执行时Oracle选择的执行方式就有不同，EMP是串行执行，而EMP_TEMP为并行执行。
官方文档:#i2231814