--------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 47183 (1)| 00:00:04 | | | |
| 1 | SORT AGGREGATE | | 1 | | | | | |
| 2 | PX COORDINATOR | | | | | | | |
| 3 | PX SEND QC (RANDOM) | :TQ10000 | 1 | | | Q1,00 | P->S | QC (RAND) |
| 4 | SORT AGGREGATE | | 1 | | | Q1,00 | PCWP | |
| 5 | PX BLOCK ITERATOR | | 21M| 47183 (1)| 00:00:04 | Q1,00 | PCWC | |
| 6 | TABLE ACCESS FULL| T1 | 21M| 47183 (1)| 00:00:04 | Q1,00 | PCWP | |
--------------------------------------------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement (level=5)
- automatic DOP: Computed Degree of Parallelism is 2
Statistics
----------------------------------------------------------
83 recursive calls
0 db block gets
470167 consistent gets
313413 physical reads
0 redo size
425 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
6 sorts (memory)
0 sorts (disk)
1 rows processed
2.parallel_min_time_threshold
parallel_min_time_threshold参数用来指定SQL语句是否并行执行一个阈值，也就是当优化器根据统计信息所估算的执行时间如果大于这个参数值就是使用并行，如果估算的执行时间小于这个参数值就会串行执行。这个参数值缺省值是10秒。并且自动并行度只要在parallel_degree_policy参数被设置为auto或limited时才会生效。从下面的信息可以看到到语句的执行时间小于10秒时，优化器以是串行而不是并行方式来执行的
SQL> show parameter parallel_degree_policy
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
parallel_degree_policy string AUTO
SQL> set autotrace on
SQL> select count(*) from t1;
COUNT(*)
----------
2755072
Elapsed: 00:00:02.66
Execution Plan
----------------------------------------------------------
Plan hash value: 3724264953
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 10627 (1)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T1 | 2569K| 10627 (1)| 00:00:01 |
-------------------------------------------------------------------
Note
-----
- dynamic sampling used for this statement (level=2)
- automatic DOP: Computed Degree of Parallelism is 1 because of parallel threshold