初试PL/SQL并行编程

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并行处理能大大提高数据的处理速度,它依赖于硬件资源、网络资源等环境.

并行处理的硬件资源环境分为5大类:

1.传统的单台计算机、单处理器、单内核的机器.(无法进行并行处理,但是并行程序还是可以运行的,此时就和普通程序一样了)

2.单台计算机、单处理器、多内核的机器.(目前本人的测试环境,没钱买多处理器的)

3.单台计算机、多处理器、多内核的机器.

4.集群系统

5.分布式系统

PS:如何查看几个处理器、几核,常用的软件就是CPU-Z.

本人的测试机器CPU:

单CPU,双核心处理器

下面开始,编写一个普通的管道函数(这里演示的就是管道函数的并行处理):

CREATE OR REPLACE FUNCTION pipe_test
(
  c_empno SYS_REFCURSOR
 ,p       VARCHAR2 DEFAULT ','
) RETURN EMP_element
  PIPELINED IS
  v_element VARCHAR2(1000);
BEGIN
  FETCH c_empno
    INTO v_element;
  LOOP
    EXIT WHEN c_empno%NOTFOUND;
    PIPE ROW(substr(v_element, 0, instr(v_element, p) - 1));
    LOOP
      v_element := substr(v_element
                         ,instr(v_element, p) + 1
                         ,length(v_element) - instr(v_element, p) + 1);
      EXIT WHEN instr(v_element, p) = 0;
      PIPE ROW(substr(v_element, 0, instr(v_element, p) - 1));
    END LOOP;
    PIPE ROW(v_element);
    FETCH c_empno
      INTO v_element;
  END LOOP;
  RETURN;
END pipe_test;

注意需要先建立类型EMP_element,如下:

CREATE OR REPLACE TYPE EMP_element as table of varchar2(100);

然后我们建立一个并行处理的管道函数:

CREATE OR REPLACE FUNCTION parallel_test
(
  p_empno SYS_REFCURSOR
 ,p       VARCHAR2 DEFAULT ','
) RETURN EMP_element
  PIPELINED
  PARALLEL_ENABLE(PARTITION p_empno BY ANY) IS
  v_element VARCHAR2(1000);
BEGIN
  FETCH p_empno
    INTO v_element;
  LOOP
    EXIT WHEN p_empno%NOTFOUND;
    PIPE ROW(substr(v_element, 0, instr(v_element, p) - 1));
    LOOP
      v_element := substr(v_element
                         ,instr(v_element, p) + 1
                         ,length(v_element) - instr(v_element, p) + 1);
      EXIT WHEN instr(v_element, p) = 0;
      PIPE ROW(substr(v_element, 0, instr(v_element, p) - 1));
    END LOOP;
    PIPE ROW(v_element);
    FETCH p_empno
      INTO v_element;
  END LOOP;
  RETURN;
END parallel_test;

留意下启用并行的关键字PARALLEL_ENABLE.

接下来我们构造大表进行测试,根据v$pq_sesstat视图的结果,我们来判断数据库系统是否真的进行了并行处理.

12:57:52 SYS@orcl> create table big_data as select 'Cc' a,'Dd' b from dual connect by level<1000000;


表已创建。

已用时间:  00: 00: 01.92
12:59:00 SYS@orcl> select count(*) from big_data;

  COUNT(*)
----------
    999999

已选择 1 行。

已用时间:  00: 00: 00.31
13:47:08 SYS@orcl> set autot trace exp stat
--普通管道函数耗时40.48秒
13:48:42 SYS@orcl> select SCOTT.pipe_test(CURSOR(SELECT a||','||b from big_data)) FROM DUAL;

已选择 1 行。

已用时间:  00: 00: 40.48

执行计划
----------------------------------------------------------
Plan hash value: 2452824241

------------------------------------------------------------------------------------------------
| Id  | Operation                          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                   |           |     1 |       |     2   (0)| 00:00:01 |
|   1 |  VIEW                              |           |  8168 |   414K|    29   (0)| 00:00:01 |
|   2 |   COLLECTION ITERATOR PICKLER FETCH| PIPE_TEST |  8168 |       |    29   (0)| 00:00:01 |
|   3 |  FAST DUAL                         |           |     1 |       |     2   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


统计信息
----------------------------------------------------------
    1000515  recursive calls
          0  db block gets
    1000325  consistent gets
       1548  physical reads
          0  redo size
    6027769  bytes sent via SQL*Net to client
       1214  bytes received via SQL*Net from client
         10  SQL*Net roundtrips to/from client
          7  sorts (memory)
          0  sorts (disk)
          1  rows processed

13:53:24 SYS@orcl> alter system flush shared_pool;

系统已更改。

已用时间:  00: 00: 01.43
--使用启用并行的管道函数,耗时40.32,与普通的管道函数耗时差不多
--奇怪的是执行计划了多了一个BIG_DATA的全表扫描操作了
13:53:26 SYS@orcl> select SCOTT.parallel_test(CURSOR(SELECT a||','||b from big_data)) FROM DUAL;

已选择 1 行。

已用时间:  00: 00: 40.32

执行计划
----------------------------------------------------------
Plan hash value: 1682567826

----------------------------------------------------------------------------------------------------

| Id  | Operation                          | Name          | Rows  | Bytes | Cost (%CPU)| Time     |

----------------------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT                   |               |     1 |       |     2   (0)| 00:00:01 |

|   1 |  VIEW                              |               |  8168 |   414K|    29   (0)| 00:00:01 |

|   2 |   COLLECTION ITERATOR PICKLER FETCH| PARALLEL_TEST |  8168 |       |    29   (0)| 00:00:01 |

|   3 |    TABLE ACCESS FULL               | BIG_DATA      |   968K|  7565K|   422   (3)| 00:00:06 |

|   4 |  FAST DUAL                         |               |     1 |       |     2   (0)| 00:00:01 |

----------------------------------------------------------------------------------------------------


Note
-----
   - dynamic sampling used for this statement (level=2)


统计信息
----------------------------------------------------------
    1000867  recursive calls
          0  db block gets
    1000353  consistent gets
       1560  physical reads
          0  redo size
    6026847  bytes sent via SQL*Net to client
       1006  bytes received via SQL*Net from client
          8  SQL*Net roundtrips to/from client
          9  sorts (memory)
          0  sorts (disk)
          1  rows processed
--通过v$pq_sesstat视图我们发现此时系统并没有进行并行处理,(其实通过执行计划也可看出没有使用并行)
13:54:21 SYS@orcl> select * from v$pq_sesstat;

STATISTIC                      LAST_QUERY SESSION_TOTAL
------------------------------ ---------- -------------
Queries Parallelized                    0             0
DML Parallelized                        0             0
DDL Parallelized                        0             0
DFO Trees                               0             0
Server Threads                          0             0
Allocation Height                       0             0
Allocation Width                        0             0
Local Msgs Sent                         0             0
Distr Msgs Sent                         0             0
Local Msgs Recv'd                       0             0
Distr Msgs Recv'd                       0             0

已选择11行。

已用时间:  00: 00: 00.00
		  
13:54:40 SYS@orcl> alter system flush buffer_cache;

系统已更改。

已用时间:  00: 00: 00.00
13:55:27 SYS@orcl> alter system flush shared_pool;

系统已更改。

已用时间:  00: 00: 01.40
--接下来我们加HINT,强制并行处理,此时耗时25.03秒,降低了一半
13:55:30 SYS@orcl> select /*+ parallel */scott.PARALLEL_TEST(CURSOR(SELECT a||','||b from big_data))
 FROM DUAL;

已选择 1 行。

已用时间:  00: 00: 25.03

执行计划
----------------------------------------------------------
Plan hash value: 3127237831

-----------------------------------------------------------------------------------------------------------------------------------

| Id  | Operation                            | Name          | Rows  | Bytes | Cost (%CPU)| Time |    TQ  |IN-OUT| PQ Distrib |

-----------------------------------------------------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT                     |               |     1 |       |     2   (0)| 00:00:01 |        |      |            |

|   1 |  PX COORDINATOR                      |               |       |       |            | |        |      |            |

|   2 |   PX SEND QC (RANDOM)                | :TQ10000      |  8168 |   414K|    16   (0)| 00:00:01 |  Q1,00 | P->S | QC (RAND)  |

|   3 |    VIEW                              |               |  8168 |   414K|    16   (0)| 00:00:01 |  Q1,00 | PCWP |            |

|   4 |     COLLECTION ITERATOR PICKLER FETCH| PARALLEL_TEST |  8168 |       |    16   (0)| 00:00:01 |  Q1,00 | PCWP |            |

|   5 |      PX BLOCK ITERATOR               |               |   968K|  7565K|   234   (3)| 00:00:03 |  Q1,00 | PCWC |            |

|   6 |       TABLE ACCESS FULL              | BIG_DATA      |   968K|  7565K|   234   (3)| 00:00:03 |  Q1,00 | PCWP |            |

|   7 |  FAST DUAL                           |               |     1 |       |     2   (0)| 00:00:01 |        |      |            |

-----------------------------------------------------------------------------------------------------------------------------------


Note
-----
   - dynamic sampling used for this statement (level=2)
   - automatic DOP: Computed Degree of Parallelism is 2


统计信息
----------------------------------------------------------
    1000941  recursive calls
          4  db block gets
    1000516  consistent gets
       1842  physical reads
          0  redo size
    6026847  bytes sent via SQL*Net to client
       1006  bytes received via SQL*Net from client
          8  SQL*Net roundtrips to/from client
          9  sorts (memory)
          0  sorts (disk)
          1  rows processed
--上面的执行计划及v$pq_sesstat都显示系统启用了并行处理,并行度为2
13:57:44 SYS@orcl> select * from v$pq_sesstat;

STATISTIC                      LAST_QUERY SESSION_TOTAL
------------------------------ ---------- -------------
Queries Parallelized                    0             1
DML Parallelized                        0             0
DDL Parallelized                        0             0
DFO Trees                               0             1
Server Threads                          0             0
Allocation Height                       0             0
Allocation Width                        0             0
Local Msgs Sent                         0           553
Distr Msgs Sent                         0             0
Local Msgs Recv'd                       0           553
Distr Msgs Recv'd                       0             0

已选择11行。

已用时间:  00: 00: 00.01

为了使示例更加有说服力,我们再来试一下普通管道函数+HINT强制并行看看:

16:02:36 SYS@orcl> select /*+ parallel */SCOTT.pipe_test(CURSOR(SELECT a||','||b from big_data)) FROM DUAL;

已选择 1 行。

已用时间:  00: 00: 43.34

执行计划
----------------------------------------------------------
Plan hash value: 2452824241

------------------------------------------------------------------------------------------------
| Id  | Operation                          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                   |           |     1 |       |     2   (0)| 00:00:01 |
|   1 |  VIEW                              |           |  8168 |   414K|    16   (0)| 00:00:01 |
|   2 |   COLLECTION ITERATOR PICKLER FETCH| PIPE_TEST |  8168 |       |    16   (0)| 00:00:01 |
|   3 |  FAST DUAL                         |           |     1 |       |     2   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)
   - automatic DOP: Computed Degree of Parallelism is 1


统计信息
----------------------------------------------------------
    1003093  recursive calls
         18  db block gets
    1000973  consistent gets
       1628  physical reads
          0  redo size
    6026843  bytes sent via SQL*Net to client
       1006  bytes received via SQL*Net from client
          8  SQL*Net roundtrips to/from client
        100  sorts (memory)
          0  sorts (disk)
          1  rows processed

由于普通函数不支持并行,所以即使+HINT强制并行,系统还是没有使用并行处理.耗时依然为40多秒.

如果不使用函数,使用普通的SQL,又是可以使用HINT强制并行的,如下:

15:59:07 SYS@orcl> select /*+ parallel */ a||','||b from big_data;

已选择999999行。

已用时间:  00: 00: 04.09

执行计划
----------------------------------------------------------
Plan hash value: 2638980575

--------------------------------------------------------------------------------------------------------------
| Id  | Operation            | Name     | Rows  | Bytes | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
--------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |          |   968K|  7565K|   234   (3)| 00:00:03 |        |      |         |
|   1 |  PX COORDINATOR      |          |       |       |            |          |        |      |         |
|   2 |   PX SEND QC (RANDOM)| :TQ10000 |   968K|  7565K|   234   (3)| 00:00:03 |  Q1,00 | P->S | QC (RAND)  |
|   3 |    PX BLOCK ITERATOR |          |   968K|  7565K|   234   (3)| 00:00:03 |  Q1,00 | PCWC |         |
|   4 |     TABLE ACCESS FULL| BIG_DATA |   968K|  7565K|   234   (3)| 00:00:03 |  Q1,00 | PCWP |         |
--------------------------------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)
   - automatic DOP: Computed Degree of Parallelism is 2


统计信息
----------------------------------------------------------
          6  recursive calls
          0  db block gets
       1577  consistent gets
       1520  physical reads
          0  redo size
   13467035  bytes sent via SQL*Net to client
     733742  bytes received via SQL*Net from client
      66668  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
     999999  rows processed

通过上面的示例可以看到,并行处理可以大大加速数据的处理,几乎成倍的提升性能.

但并不是并行度为几,就会提示几倍的性能,它受硬件环境,优化器内部算法,PL/SQL引擎解析等等的约束.

总之,并行处理给我们带来的性能提升是很大的,所以在某些场景使用并行编程也是很必要的.