race 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:http://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_2013.htm#i2231814