dml并行

Enabling Parallel DML
A DML statement can be parallelized only if you have explicitly enabled parallel DML in the session, as in the following statement:
关于dml并行必须在会话层面明确指定，不指定都是位并行

ALTER SESSION ENABLE PARALLEL DML;

下面查看例子

create table scott.t1 as select b.OBJECT_ID,b.OBJECT_NAME from dba_objects b ;
 SQL> select count(1) from scott.t1;

  COUNT(1)
----------
   608657

SQL> update /*+ parallel(a,2) */  scott.t1 a set OBJECT_NAME='hthorizon.com';

608657 rows updated.

Elapsed: 00:00:37.12

alter system flush buffer_cache;
alter system flush shared_pool;

SQL> alter session enable parallel dml;

Session altered.

Elapsed: 00:00:00.03
SQL> update /*+ parallel(a,2) */  scott.t1 a set OBJECT_NAME='hthorizon.com1';

608657 rows updated.

Elapsed: 00:00:13.56
真正的 parallel update更新比伪parallel块0.24秒，下面查看执行计划

 SQL> explain plan for update /*+ parallel(a,2) */  scott.t1 a set OBJECT_NAME='hthorizon.com';

Explained.

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

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------------------
Plan hash value: 121765358

---------------------------------------------------------------------------------------------------------------
| Id  | Operation	      | Name	 | Rows  | Bytes | Cost (%CPU)| Time	 |    TQ  |IN-OUT| PQ Distrib |
---------------------------------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT      | 	 |  1391K|    33M|  1011   (1)| 00:00:13 |	  |	 |	      |
|   1 |  UPDATE 	          | T1	 |	 |	 |	      | 	 |	  |	 |	      |
|   2 |   PX COORDINATOR      | 	 |	 |	 |	      | 	 |	  |	 |	      |
|   3 |    PX SEND QC (RANDOM)| :TQ10000 |  1391K|    33M|  1011   (1)| 00:00:13 |  Q1,00 | P->S | QC (RAND)  |
|   4 |     PX BLOCK ITERATOR | 	 |  1391K|    33M|  1011   (1)| 00:00:13 |  Q1,00 | PCWC |	      |
|   5 |      TABLE ACCESS FULL| T1	 |  1391K|    33M|  1011   (1)| 00:00:13 |  Q1,00 | PCWP |	      |

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------

上面并行更新时伪并行，只有在select时并行，真正的更新并行是发生在P->S（parallel_to_serial）之后;更新慢是慢在QC（query coordinator/查询协调)之上
SQL> alter session enable parallel dml;

Session altered.

SQL> explain plan for update /*+ parallel(a,2) */  scott.t1 a set OBJECT_NAME='hthorizon.com';

Explained.

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

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3991856572

---------------------------------------------------------------------------------------------------------------
| Id  | Operation	      | Name	 | Rows  | Bytes | Cost (%CPU)| Time	 |    TQ  |IN-OUT| PQ Distrib |
---------------------------------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT      | 	 |  1391K|    33M|  1011   (1)| 00:00:13 |	  |	 |	      |
|   1 |  PX COORDINATOR       | 	 |	 |	 |	      | 	 |	  |	 |	      |
|   2 |   PX SEND QC (RANDOM) | :TQ10000 |  1391K|    33M|  1011   (1)| 00:00:13 |  Q1,00 | P->S | QC (RAND)  |
|   3 |    UPDATE	          | T1	 |	 |	 |	      | 	 |  Q1,00 | PCWP |	      |
|   4 |     PX BLOCK ITERATOR | 	 |  1391K|    33M|  1011   (1)| 00:00:13 |  Q1,00 | PCWC |	      |
|   5 |      TABLE ACCESS FULL| T1	 |  1391K|    33M|  1011   (1)| 00:00:13 |  Q1,00 | PCWP |	      |

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------

从上述执行计划来看是真正的update并行
查看sql的实际执行计划（执行过的语句才能生成真实的执行计划）
SQL> select a.SQL_ID,a.EXECUTIONS,a.CHILD_number from v$sql a where a.SQL_TEXT like 'update /*+ parallel(a,2) */  scott.t1%';

SQL_ID	      EXECUTIONS CHILD_NUMBER
------------- ---------- ------------
9fwr6kw5xwmq0	       1	    0

Elapsed: 00:00:00.00

SQL> select * from table(xplan.DISPLAY_cursor('9fwr6kw5xwmq0',0,'advanced')); 

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID	9fwr6kw5xwmq0, child number 0
-------------------------------------
update /*+ parallel(a,2) */  scott.t1 a set OBJECT_NAME='hthorizon.com1'

Plan hash value: 3991856572

-----------------------------------------------------------------------------------------------------------------------
| Id  | Order | Operation	      | Name	 | Rows  | Bytes | Cost (%CPU)| Time	 |    TQ  |IN-OUT| PQ Distrib |
-----------------------------------------------------------------------------------------------------------------------
|   0 |     6 | UPDATE STATEMENT      | 	 |	 |	 |  1011 (100)| 	 |	  |	 |	      |
|   1 |     5 |  PX COORDINATOR       | 	 |	 |	 |	      | 	 |	  |	 |	      |

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
|   2 |     4 |   PX SEND QC (RANDOM) | :TQ10000 |  1391K|    33M|  1011   (1)| 00:00:13 |  Q1,00 | P->S | QC (RAND)  |
|   3 |     3 |    UPDATE	      | T1	 |	 |	 |	      | 	 |  Q1,00 | PCWP |	      |
|   4 |     2 |     PX BLOCK ITERATOR | 	 |  1391K|    33M|  1011   (1)| 00:00:13 |  Q1,00 | PCWC |	      |
|*  5 |     1 |      TABLE ACCESS FULL| T1	 |  1391K|    33M|  1011   (1)| 00:00:13 |  Q1,00 | PCWP |	      |
-----------------------------------------------------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
---------------------------------------------------------------------

   1 - UPD$1
   5 - UPD$1 / A@UPD$1

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------

Outline Data
-------------

  /*+
      BEGIN_OUTLINE_DATA
      IGNORE_OPTIM_EMBEDDED_HINTS
      OPTIMIZER_FEATURES_ENABLE('11.2.0.4')
      DB_VERSION('11.2.0.4')
      ALL_ROWS
      OUTLINE_LEAF(@"UPD$1")

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
      FULL(@"UPD$1" "A"@"UPD$1")
      END_OUTLINE_DATA
  */

Predicate Information (identified by operation id):
---------------------------------------------------

   5 - access(:Z>=:Z AND :Z<=:Z)

Column Projection Information (identified by operation id):
-----------------------------------------------------------

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------

   1 - SYSDEF[4], SYSDEF[32720], SYSDEF[1], SYSDEF[96], SYSDEF[32720]
   2 - (#keys=0) SYSDEF[4], SYSDEF[32720], SYSDEF[1], SYSDEF[96], SYSDEF[32720]
   3 - SYSDEF[4], SYSDEF[32720], SYSDEF[1], SYSDEF[96], SYSDEF[32720]
   4 - (upd=2) "A".ROWID[ROWID,10], "OBJECT_NAME"[VARCHAR2,128]
   5 - "A".ROWID[ROWID,10], "OBJECT_NAME"[VARCHAR2,128]

51 rows selected.