[20171225]查看并行执行安顿注意的问题.txt

[20171225]翻看并行执行布署注意的问题.txt

–//如若使用dbms_xplan.display_cursor查看并行执行布署注意一些题材,通过例子表明:

1.环境:

SCOTT@book> @ &r/ver1
PORT_STRING                    VERSION        BANNER



x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g
Enterprise Edition Release 11.2.0.4.0 – 64bit Production

2.测试:
SCOTT@book> create table t1 as select * from dba_objects ;
Table created.

SCOTT@book> alter session set statistics_level=all;
Session altered.

–//分析表略.

SCOTT@book> select /*+ parallel(t1,4) */ count(*) from t1;

  COUNT(*)

     87016

SCOTT@book> @ &r/dpc ” ”

PLAN_TABLE_OUTPUT

SQL_ID  6yhkc72j9mnnt, child number 0

select /*+ parallel(t1,4) */ count(*) from t1

Plan hash value: 3110199320

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

Buffers | Reads  |

|   0 | SELECT STATEMENT       |          |      1 |        |    96
(100)|          |        |      |            |      1 |00:00:00.02
|       5 |      1 |
|   1 |  SORT AGGREGATE        |          |      1 |      1 |           
|          |        |      |            |      1 |00:00:00.02 |       5
|      1 |
|   2 |   PX COORDINATOR       |          |      1 |        |           
|          |        |      |            |      4 |00:00:00.02 |       5
|      1 |
|   3 |    PX SEND QC (RANDOM) | :TQ10000 |      0 |      1 |           
|          |  Q1,00 | P->S | QC (RAND)  |      0 |00:00:00.01 |      
0 |      0 |
|   4 |     SORT AGGREGATE     |          |      0 |      1 |           
|          |  Q1,00 | PCWP |            |      0 |00:00:00.01 |       0
|      0 |
|   5 |      PX BLOCK ITERATOR |          |      0 |  87016 |    96  
(0)| 00:00:02 |  Q1,00 | PCWC |            |      0 |00:00:00.01 |      
0 |      0 |
|*  6 |       TABLE ACCESS FULL| T1       |      0 |  87016 |    96  
(0)| 00:00:02 |  Q1,00 | PCWP |            |      0 |00:00:00.01 |      

0 |      0 |

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

   1 – SEL$1
   6 – SEL$1 / T1@SEL$1

Predicate Information (identified by operation id):

   6 – access(:Z>=:Z AND :Z<=:Z)

–//全表扫描,然则注意看A-Rows实际上根本不对.看到是0行.而E-Rows看到是科学的.

SCOTT@book> select * from
table(dbms_xplan.display_cursor(‘6yhkc72j9mnnt’,NULL,’ALLSTATS LAST
PEEKED_BINDS cost partition -projection -outline parallel’));

PLAN_TABLE_OUTPUT

SQL_ID  6yhkc72j9mnnt, child number 0

select /*+ parallel(t1,4) */ count(*) from t1

Plan hash value: 3110199320

| Id  | Operation              | Name     | Starts | E-Rows | Cost

(%CPU)|    TQ  |IN-OUT| PQ Distrib | A-Rows |   A-Time   | Buffers |

|   0 | SELECT STATEMENT       |          |      1 |        |    96
(100)|        |      |            |      1 |00:00:00.11 |       5 |
|   1 |  SORT AGGREGATE        |          |      1 |      1 |           
|        |      |            |      1 |00:00:00.11 |       5 |
|   2 |   PX COORDINATOR       |          |      1 |        |           
|        |      |            |      4 |00:00:00.11 |       5 |
|   3 |    PX SEND QC (RANDOM) | :TQ10000 |      0 |      1 |           
|  Q1,00 | P->S | QC (RAND)  |      0 |00:00:00.01 |       0 |
|   4 |     SORT AGGREGATE     |          |      0 |      1 |           
|  Q1,00 | PCWP |            |      0 |00:00:00.01 |       0 |
|   5 |      PX BLOCK ITERATOR |          |      0 |  87016 |    96  
(0)|  Q1,00 | PCWC |            |      0 |00:00:00.01 |       0 |
|*  6 |       TABLE ACCESS FULL| T1       |      0 |  87016 |    96  

(0)|  Q1,00 | PCWP |            |      0 |00:00:00.01 |       0 |

Predicate Information (identified by operation id):

   6 – access(:Z>=:Z AND :Z<=:Z)
–//出席parallel提醒也是一样.
–//链接:raajeshwaran.blogspot.com/2017/12/gatherplanstatistics-hint-for-parallel.html
In a parallel execution the last process to execute the cursor is the
Query coordinator (QC), typically this QC will
execute a small number of operations in the execution plan, while the
majority of the operations in the plan was done by
the parallel execution server process. So when we issue the
DBMS_XPLAN.DISPLAY_CURSOR and ask for the last execution we
only get the information about the operations in the plan that the QC
actually executed. In this case the only operation
that QC did was return the final result to our SQL*Plus session, which
is why the line 0 and 1 and 2 have entries in the
A-rows column.

In order to see A-rows values for all the operations in the plan, we
have to use the FORMAT value as ALLSTATS ALL, which
will show you the execution statistics for ALL executions of the
cursor.

SCOTT@book> select * from
table(dbms_xplan.display_cursor(‘6yhkc72j9mnnt’,NULL,’ALLSTATS ALL
PEEKED_BINDS cost partition -projection -outline parallel’));

PLAN_TABLE_OUTPUT

SQL_ID  6yhkc72j9mnnt, child number 0

select /*+ parallel(t1,4) */ count(*) from t1

Plan hash value: 3110199320

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

Buffers | Reads  |

|   0 | SELECT STATEMENT       |          |      2 |        |    96
(100)|          |        |      |            |      2 |00:00:00.13
|      10 |      1 |
|   1 |  SORT AGGREGATE        |          |      2 |      1 |           
|          |        |      |            |      2 |00:00:00.13 |      10
|      1 |
|   2 |   PX COORDINATOR       |          |      2 |        |           
|          |        |      |            |      8 |00:00:00.13 |      10
|      1 |
|   3 |    PX SEND QC (RANDOM) | :TQ10000 |      0 |      1 |           
|          |  Q1,00 | P->S | QC (RAND)  |      0 |00:00:00.01 |      
0 |      0 |
|   4 |     SORT AGGREGATE     |          |      7 |      1 |           
|          |  Q1,00 | PCWP |            |      7 |00:00:00.10 |    2265
|   2179 |
|   5 |      PX BLOCK ITERATOR |          |      8 |  87016 |    96  
(0)| 00:00:02 |  Q1,00 | PCWC |            |    152K|00:00:00.09 |   
2590 |   2486 |
|*  6 |       TABLE ACCESS FULL| T1       |    104 |  87016 |    96  
(0)| 00:00:02 |  Q1,00 | PCWP |            |    174K|00:00:00.04 |   

2590 |   2486 |

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

   1 – SEL$1
   6 – SEL$1 / T1@SEL$1

Predicate Information (identified by operation id):

   6 – access(:Z>=:Z AND :Z<=:Z)

–//而这里看到的A-Rows实际上往往执行后的聚积,并不可以感应真实的情形.使用参数all的状态导致的结果.

–//出席提醒,生成新的履行安插:
SCOTT@book> select /*+ parallel(t1,4) test */ count(*) from t1;

  COUNT(*)

     87016

SCOTT@book> select * from
table(dbms_xplan.display_cursor(NULL,NULL,’ALLSTATS ALL PEEKED_BINDS
cost partition -projection -outline parallel’));

PLAN_TABLE_OUTPUT

SQL_ID  6a3vj021614ft, child number 0

select /*+ parallel(t1,4) test */ count(*) from t1

Plan hash value: 3110199320

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

Buffers | Reads  |

|   0 | SELECT STATEMENT       |          |      1 |        |    96
(100)|          |        |      |            |      1 |00:00:00.11
|       5 |      0 |
|   1 |  SORT AGGREGATE        |          |      1 |      1 |           
|          |        |      |            |      1 |00:00:00.11 |       5
|      0 |
|   2 |   PX COORDINATOR       |          |      1 |        |           
|          |        |      |            |      4 |00:00:00.11 |       5
|      0 |
|   3 |    PX SEND QC (RANDOM) | :TQ10000 |      0 |      1 |           
|          |  Q1,00 | P->S | QC (RAND)  |      0 |00:00:00.01 |      
0 |      0 |
|   4 |     SORT AGGREGATE     |          |      4 |      1 |           
|          |  Q1,00 | PCWP |            |      4 |00:00:00.06 |    1295
|   1243 |
|   5 |      PX BLOCK ITERATOR |          |      4 |  87016 |    96  
(0)| 00:00:02 |  Q1,00 | PCWC |            |  87016 |00:00:00.05 |   
1295 |   1243 |
|*  6 |       TABLE ACCESS FULL| T1       |     52 |  87016 |    96  
(0)| 00:00:02 |  Q1,00 | PCWP |            |  87016 |00:00:00.02 |   

1295 |   1243 |

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

   1 – SEL$1
   6 – SEL$1 / T1@SEL$1

Predicate Information (identified by operation id):

   6 – access(:Z>=:Z AND :Z<=:Z)

–//这样看到的实施安排才是相比较实在的数值.

–//我的dpc.sql脚本如下:
select * from
table(dbms_xplan.display_cursor(NVL(‘&1′,NULL),NULL,’ALL ALLSTATS LAST
PEEKED_BINDS cost partition -projection -outline &2’));
–//我写的脚本也存在问题,可是最终的last掩盖前面all参数的设置.^_^.

3.总结:
–//在设置statistics_level=all;或者升迁gather_plan_statistics时,看到的并行执行安插要更加注意.

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