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db_file_multiblock_read_count参数设置取值测试

王朝other·作者佚名  2006-11-22
窄屏简体版  字體: |||超大  

1.创建测试表

create table big_table as select * from all_objects;

2.获取等待时间

create or replace procedure full_scan_big_table(p_dbmrc in number)

as

l_cnt number;

begin

execute immediate 'alter tablespace users offline';

execute immediate 'alter tablespace users online';

execute immediate 'alter session set db_file_multiblock_read_count=' || p_dbmrc;

execute immediate 'alter session set events ''10046 trace name context forever, level 12'' ';

execute immediate 'select /*+ FULL(bt_mbrc_'|| p_dbmrc || ') */ count(*) from big_table bt_mbrc_'||p_dbmrc into l_cnt;

execute immediate 'alter session set events ''10046 trace name context off'' ';

end;

执行如下脚本,对trc文件运行tkprof,可以得到不同db_file_multiblock_read_count设置时执行类似

select /*+ FULL(b) */ count(*) from big_table b;查询的等待时间:

begin

for i in 0..9

loop

full_scan_big_table(power(2,i));

end loop;

end;

3.测试查询成本

set autotrace traceonly explain

alter session set db_file_multiblock_read_count=4;

select /*+ FULL(b) */ count(*) from big_table b;

执行结果:

SQL> alter session set db_file_multiblock_read_count=4;

Session altered.

SQL> select /*+ FULL(b) */ count(*) from big_table b;

Execution Plan

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

Plan hash value: 3812947622

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

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

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

| 0 | SELECT STATEMENT | | 1 | 280 (6)| 00:00:04 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | TABLE ACCESS FULL| BIG_TABLE | 51035 | 280 (6)| 00:00:04 |

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

alter session set db_file_multiblock_read_count=8;

select /*+ FULL(b) */ count(*) from big_table b;

执行结果:

SQL> alter session set db_file_multiblock_read_count=8;

Session altered.

SQL> select /*+ FULL(b) */ count(*) from big_table b;

Execution Plan

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

Plan hash value: 3812947622

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

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

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

| 0 | SELECT STATEMENT | | 1 | 207 (8)| 00:00:03 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | TABLE ACCESS FULL| BIG_TABLE | 51035 | 207 (8)| 00:00:03 |

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

alter session set db_file_multiblock_read_count=16;

select /*+ FULL(b) */ count(*) from big_table b;

执行结果:

SQL> alter session set db_file_multiblock_read_count=16;

Session altered.

SQL> select /*+ FULL(b) */ count(*) from big_table b;

Execution Plan

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

Plan hash value: 3812947622

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

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

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

| 0 | SELECT STATEMENT | | 1 | 170 (9)| 00:00:03 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | TABLE ACCESS FULL| BIG_TABLE | 51035 | 170 (9)| 00:00:03 |

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

alter session set db_file_multiblock_read_count=32;

select /*+ FULL(b) */ count(*) from big_table b;

执行结果:

SQL> alter session set db_file_multiblock_read_count=32;

Session altered.

SQL> select /*+ FULL(b) */ count(*) from big_table b;

Execution Plan

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

Plan hash value: 3812947622

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

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

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

| 0 | SELECT STATEMENT | | 1 | 152 (10)| 00:00:02 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | TABLE ACCESS FULL| BIG_TABLE | 51035 | 152 (10)| 00:00:02 |

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

alter session set db_file_multiblock_read_count=64;

select /*+ FULL(b) */ count(*) from big_table b;

执行结果:

SQL> alter session set db_file_multiblock_read_count=64;

Session altered.

SQL> select /*+ FULL(b) */ count(*) from big_table b;

Execution Plan

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

Plan hash value: 3812947622

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

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

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

| 0 | SELECT STATEMENT | | 1 | 143 (11)| 00:00:02 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | TABLE ACCESS FULL| BIG_TABLE | 51035 | 143 (11)| 00:00:02 |

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

alter session set db_file_multiblock_read_count=128;

select /*+ FULL(b) */ count(*) from big_table b;

执行结果:

SQL> alter session set db_file_multiblock_read_count=128;

Session altered.

SQL> select /*+ FULL(b) */ count(*) from big_table b;

Execution Plan

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

Plan hash value: 3812947622

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

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

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

| 0 | SELECT STATEMENT | | 1 | 138 (11)| 00:00:02 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | TABLE ACCESS FULL| BIG_TABLE | 51035 | 138 (11)| 00:00:02 |

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

alter session set db_file_multiblock_read_count=256;

select /*+ FULL(b) */ count(*) from big_table b;

执行结果:

SQL> alter session set db_file_multiblock_read_count=256;

Session altered.

SQL> select /*+ FULL(b) */ count(*) from big_table b;

Execution Plan

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

Plan hash value: 3812947622

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

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

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

| 0 | SELECT STATEMENT | | 1 | 138 (11)| 00:00:02 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | TABLE ACCESS FULL| BIG_TABLE | 51035 | 138 (11)| 00:00:02 |

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

alter session set db_file_multiblock_read_count=512;

select /*+ FULL(b) */ count(*) from big_table b;

SQL> alter session set db_file_multiblock_read_count=512;

Session altered.

SQL> select /*+ FULL(b) */ count(*) from big_table b;

Execution Plan

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

Plan hash value: 3812947622

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

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

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

| 0 | SELECT STATEMENT | | 1 | 138 (11)| 00:00:02 |

| 1 | SORT AGGREGATE | | 1 | | |

| 2 | TABLE ACCESS FULL| BIG_TABLE | 51035 | 138 (11)| 00:00:02 |

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

观察不同db_file_multiblock_read_count参数设置下:查询成本/1000+等待时间

可以得出:最好的db_file_multiblock_read_count值应该为16,该参数取值的阈

值依赖于操作系统和硬件,应该对不同的操作系统和硬件,多作测试。

 
 
 
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