Oracle11gR2 全表扫描成本计算(工作量模式-workload)

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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

SQL> alter session set db_file_multiblock_read_count=32;

Session altered.

SQL> select count(*) from test;

Execution Plan
----------------------------------------------------------
Plan hash value: 1950795681

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。

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