Oracle11gR2 全表扫描成本计算(工作量模式-workload)

原文链接：http://blog.csdn.net/robinson1988/article/details/6606103

昨天测试了非工作量模式下Oracle11gR2全表扫描的成本计算，现在测试一下在工作量模式下Oracle11gR2全表扫描的成本计算

首先讲表blocks增加到10003个

SQL> select owner,blocks from dba_tables where table_name='TEST' and owner='TEST';

OWNER                              BLOCKS

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

TEST                                10003

然后人工设置工作量的CPUSPEED=2500，单块读等于5，多块读等于30，MBRC等于12

SQL> begin

   dbms_stats.set_system_stats('CPUSPEED',2500);

   dbms_stats.set_system_stats('SREADTIM',5);

   dbms_stats.set_system_stats('MREADTIM',30);

   dbms_stats.set_system_stats('MBRC',12);

end;

/  2    3    4    5    6    7

PL/SQL procedure successfully completed.

利用explain plan得到CPU_COST---这里等于 72735764

SQL> explain plan for select count(*) from test;

Explained.

SQL> select cpu_cost from plan_table;

  CPU_COST

----------

  72735764

成本计算公式如下：

Cost = (

       #SRds * sreadtim +

       #MRds * mreadtim +

       CPUCycles / cpuspeed /1000

       ) / sreadtime

       

#SRds - number of single block reads

#MRds - number of multi block reads

#CPUCyles - number of CPU cycles

sreadtim - single block read time

mreadtim - multi block read time

cpuspeed - CPU cycles per second

Cost = (

       #SRds * sreadtim +                            ---SRds=0

       #MRds * mreadtim +                          ---MRds=BLOCKS/MBCR=10003/12, mreadtim=30

       CPUCycles / cpuspeed / 1000         ---CPUCycles=PLAN_TABLE.CPU_COST,cpuspeed=2500

       ) / sreadtime

所以人工计算的成本等于：

SQL> select ceil(10003/12*30/5)+ceil(72735764/2500/5/1000)+1 from dual;

CEIL(10003/12*30/5)+CEIL(72735764/2500/5/1000)+1

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

                                            5009

SQL> set autot trace

SQL> select count(*) from test;

Execution Plan

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

Plan hash value: 1950795681

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

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

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

|   0 | SELECT STATEMENT   |      |     1 |  5009   (1)| 00:00:26 |

|   1 |  SORT AGGREGATE    |      |     1 |            |          |

|   2 |   TABLE ACCESS FULL| TEST | 10000 |  5009   (1)| 00:00:26 |

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

人工计算的cost正好等于Oracle计算的Cost 这里也说明Oracle11gR2 在工作量模式下，全表扫描的成本计算方法依然同Oracle9i,Oracle10g

工作量模式下，从全表扫描的成本可以看出，参数db_file_multiblock_read_count 的更改对全表扫描成本计算没有影响，有影响的是MBRC，举个例子：

SQL> show parameter db_file_multiblock_read_count

NAME                                 TYPE        VALUE

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

db_file_multiblock_read_count        integer     16

SQL> set autot trace

SQL> select count(*) from test;

Execution Plan

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

Plan hash value: 1950795681

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

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

-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |  5009   (1)| 00:00:26 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |

|   2 |   TABLE ACCESS FULL| TEST | 10000 |  5009   (1)| 00:00:26 |

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

SQL> alter session set db_file_multiblock_read_count=32;

Session altered.

SQL>  select count(*) from test;

Execution Plan

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

Plan hash value: 1950795681

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

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

-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |  5009   (1)| 00:00:26 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |

|   2 |   TABLE ACCESS FULL| TEST | 10000 |  5009   (1)| 00:00:26 |

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

可以看到更改db_file_multiblock_read_count对于成本没有任何影响，因为工作量模式下的COST只跟MBRC有关。