oracle實驗記錄 （全表掃描COST計算方法）
實驗下FTS 時候 COST 的計算
公式:
According to the CPU costing model:
Cost = (
#SRds * sreadtim +
#MRds * mreadtim +~~~~~~~~~~~~~~~~~~~~~~~~~~~~來自於cost-based oracle書中
#CPUCycles / cpuspeed
) / sreadtim
where
#SRDs - number of single block reads
#MRDs - number of multi block reads
#CPUCycles - number of CPU Cycles
sreadtim - single block read time
mreadtim - multi block read time
cpuspeed - CPU cycles per second
COST=CPU COST+IO COST (8I 不計算CPU COST)
SQL> select * from v$version;實驗版本
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
測試
SQL> create table t1 (a int);
表已建立。
SQL> ed
已寫入 file afiedt.buf
1 declare
2 begin
3 for i in 1..10000 loop
4 insert into t1 values(i);
5 end loop;
6* end;
SQL> /
PL/SQL 過程已成功完成。
SQL> select blocks from user_tables where table_name='T1';
BLOCKS
----------
SQL> exec dbms_stats.gather_table_stats('SYS','T1');
PL/SQL 過程已成功完成。
SQL> select blocks from user_tables where table_name='T1';
BLOCKS
----------
20
SQL> show parameter db_block_size
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_block_size integer 8192
SQL> show parameter db_file_mu
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_file_multiblock_read_count integer 16
SQL> alter session set "_optimizer_cost_model"=CPU;
會話已更改。
SQL> set autotrace traceonly
SQL> alter session set events '10053 trace name context forever';
會話已更改。
SQL> select * from t1;
已選擇10000行。
執行計劃
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 30000 | 6 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T1 | 10000 | 30000 | 6 (0)| 00:00:01 |
--------------------------------------------------------------------------
統計資訊
----------------------------------------------------------
135 recursive calls
0 db block gets
703 consistent gets
0 physical reads
0 redo size
176812 bytes sent via SQL*Net to client
7711 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
SQL> set autotrace traceonly
SQL> alter session set events '10053 trace name context forever';~~~檢視CBO計劃
會話已更改。
SQL> select * from t1;
已選擇10000行。
執行計劃
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 30000 | 6 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T1 | 10000 | 30000 | 6 (0)| 00:00:01 |
--------------------------------------------------------------------------
統計資訊
----------------------------------------------------------
0 recursive calls
0 db block gets
690 consistent gets
0 physical reads
0 redo size
176812 bytes sent via SQL*Net to client
7711 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
SINGLE TABLE ACCESS PATH
Table: T1 Alias: T1
Card: Original: 10000 Rounded: 10000 Computed: 10000.00 Non Adjusted: 10000.00
Access Path: TableScan
Cost: 6.28 Resp: 6.28 Degree: 0
Cost_io: 6.00 Cost_cpu: 1642429~~~~~~~~~~~~~~~~~~~~~~IO部分cost=6
Resp_io: 6.00 Resp_cpu: 1642429
Best:: AccessPath: TableScan
Cost: 6.28 Degree: 1 Resp: 6.28 Card: 10000.00 Bytes: 0
**********Cost_io: 6.00 IO 部分 ,CPU=0.28
SQL> select pname,pval1 from aux_stats$ where sname='SYSSTATS_MAIN';
PNAME PVAL1
------------------------------ ----------
CPUSPEED
CPUSPEEDNW 484.974958 speed in millions of operations per second
IOSEEKTIM 10 disk seek time in milliseconds
IOTFRSPEED 4096 disk transfer time in bytes per millisecond
MAXTHR
MBRC
MREADTIM
SLAVETHR
SREADTIM
已選擇9行。以上沒有資訊收集 ORACLE 會用DEFAULT 計算************
計算*使用 預設資訊**************************
MBRC=db_file_multiblock_read_count=16
sreadtim=IOSEEKTIM+db+block_size/IOTFRSPEED=12
mreadtime=IOSEEKTIM+db_file_multiblock_read_count*db+block_size/IOTFRSPEED=42
MDRS=BLOCKS/MBRC
#SRds =0(single-block reads),因為是 FTS 都是多塊讀取
最早的公式變換一下(除開)
Cost = (
#SRds +
#MRds * mreadtim / sreadtim +**************io cost
#CPUCycles / (cpuspeed * sreadtim)********cpu cost
SQL> select 20/16*42/12 from dual;
20/16*42/12
-----------
4.375
4.375=5 由於
_optimizer_ceil_cost TRUE TRUE FALSE FALSE
CEIL cost in CBO~~~~~~~~~~取整
SQL> select ceil(20/16*42/12) from dual;
CEIL(20/16*42/12)
-----------------
5~~~~~~~~~~~~~~~~~~~~~~~~~~~IO 部分IO COST
又由於 _table_scan_cost_plus_one = true 加1 所以IO COST=6
true時候COST+1 表示開始需要訪問表的SEGMENT HEADER BLOCK,這是優化器的內建技巧之一(避免小表中用INDEX)
********oracle cost-based oracle原文描述 _table_scan_cost_plus_one
This is the type of minor tweak that appears surprisingly frequently in Oracle, and makes
it very difficult to produce any sort of documentation about the optimizer that is both concise
and complete. In this case, the change may represent the requirement for the necessary access
to the table’s segment header block at the start of a tablescan; alternatively it may simply be
one of several tricks built into the optimizer to bias it towards using indexed access paths on
very small tables.
***********
CPU 部分 轉換成IO COST 單位
#CPUCycles / (cpuspeed * sreadtim)
#CPUCycles:1642429
cpuspeed default 使用 CPUSPEEDNW
sreadtime:12(milliseconds)=12000(microseconds)
SQL> select 1642429/(484*12000) from dual;
1642429/(484*12000)
-------------------
.282787362
*********************
實驗改_table_scan_cost_plus_on為FALSE
SQL> alter system set "_table_scan_cost_plus_one"=false;
系統已更改。
SQL> set autotrace traceonly
SQL> alter session set events '10053 trace name context forever';
會話已更改。
SQL> select * from t1;
已選擇10000行。
執行計劃
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 30000 | 5 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T1 | 10000 | 30000 | 5 (0)| 00:00:01 |
--------------------------------------------------------------------------
統計資訊
----------------------------------------------------------
1 recursive calls
0 db block gets
690 consistent gets
0 physical reads
0 redo size
176812 bytes sent via SQL*Net to client
7711 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
trace中
_table_scan_cost_plus_one = false
Table Stats::
Table: T1 Alias: T1
#Rows: 10000 #Blks: 20 AvgRowLen: 3.00
***************************************
SINGLE TABLE ACCESS PATH
Table: T1 Alias: T1
Card: Original: 10000 Rounded: 10000 Computed: 10000.00 Non Adjusted: 10000.00
Access Path: TableScan
Cost: 5.28 Resp: 5.28 Degree: 0
Cost_io: 5.00 Cost_cpu: 1642429
Resp_io: 5.00 Resp_cpu: 1642429
Best:: AccessPath: TableScan
Cost: 5.28 Degree: 1 Resp: 5.28 Card: 10000.00 Bytes: 0
可以看出 IO COST=5 CPU=0.28
*************************
SQL> alter system set "_optimizer_ceil_cost"=false;
發現改過這個引數後 oracle 會出現些混亂
系統已更改。
NAME VALUE ISDEFAULT ISMOD ISADJ
------------------------------ ------------------------- --------- ---------- -----
KSPPDESC
-----------------------------------------------------------------------------------
-------------------------------------------------
_optimizer_ceil_cost FALSE FALSE FALSE FALSE
CEIL cost in CBO
SQL> /
輸入 par 的值: table_scan_cost_plus_one
原值 14: x.ksppinm like '%_&par%'
新值 14: x.ksppinm like '%_table_scan_cost_plus_one%'
NAME VALUE ISDEFAULT ISMOD ISADJ
------------------------------ ------------------------- --------- ---------- -----
KSPPDESC
-----------------------------------------------------------------------------------
-------------------------------------------------
_table_scan_cost_plus_one FALSE FALSE FALSE FALSE
bump estimated full table scan and index ffs cost by one
SQL> set autotrace traceonly
SQL> alter session set events '10053 trace name context forever';
會話已更改。
SQL> select * from t1;
已選擇10000行。
執行計劃
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 30000 | 6 (17)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T1 | 10000 | 30000 | 6 (17)| 00:00:01 |
--------------------------------------------------------------------------
統計資訊
----------------------------------------------------------
154 recursive calls
0 db block gets
706 consistent gets
23 physical reads
0 redo size
176812 bytes sent via SQL*Net to client
7711 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
10000 rows processed
SQL> exit
_table_scan_cost_plus_one = false
Table Stats::
Table: T1 Alias: T1
#Rows: 10000 #Blks: 20 AvgRowLen: 3.00
***************************************
SINGLE TABLE ACCESS PATH
Table: T1 Alias: T1
Card: Original: 10000 Rounded: 10000 Computed: 10000.00 Non Adjusted: 10000.00
Access Path: TableScan
Cost: 5.28 Resp: 5.28 Degree: 0
Cost_io: 5.00 Cost_cpu: 1642429
Resp_io: 5.00 Resp_cpu: 1642429
Best:: AccessPath: TableScan
Cost: 6.00 Degree: 1 Resp: 5.28 Card: 10000.00 Bytes: 0 注意cost Cost: 6.00 Degree: 1(並行度) Resp: 5.28(Cost for full parallel execution of a step.) 這個是指並行時候的COST ,現在並行度為1 就代表串 行 ,oracle向上取整COST 到了 6(將總5.28) 原來是5.28 autotrace中 顯示為5 現在為6
理應 為4才對 不取整 不加一 IO COST應該等於4 ，而實驗結果有點奇怪
SQL> alter system set "_optimizer_ceil_cost"=true;(這個引數只在CBO COST計算時 使用CPU_costing時候 才有用)
系統已更改。
SQL> set autotrace traceonly
SQL> alter session set events '10053 trace name context forever';
會話已更改。
SQL> select * from t1;
已選擇10000行。
執行計劃
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 30000 | 5 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T1 | 10000 | 30000 | 5 (0)| 00:00:01 |
--------------------------------------------------------------------------
統計資訊
----------------------------------------------------------
0 recursive calls
0 db block gets
690 consistent gets
0 physical reads
0 redo size
176812 bytes sent via SQL*Net to client
7711 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
SQL> alter session set events '10053 trace name context off'; 改回後正常了
會話已更改。
****************************模擬下oracle 8i 傳統COST計算 (只計算IO部分)
SQL> alter system set "_table_scan_cost_plus_one"=true;
系統已更改。
SQL> alter system set "_optimizer_cost_model"=io~~~~~~~~~~~~~換成傳統IO成本計算~~這將只單一的結算IO COST 並且不使用系統統計資訊
2 ;
系統已更改。
SQL> set autotrace traceonly
SQL> alter session set events '10053 trace name context forever';
會話已更改。
SQL> select * from t1;
已選擇10000行。
執行計劃
----------------------------------------------------------
Plan hash value: 3617692013
----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 30000 | 4 |
| 1 | TABLE ACCESS FULL| T1 | 10000 | 30000 | 4 |
----------------------------------------------------------
Note
-----
- cpu costing is off (consider enabling it)
統計資訊
----------------------------------------------------------
1 recursive calls
0 db block gets
690 consistent gets
0 physical reads
0 redo size
176812 bytes sent via SQL*Net to client
7711 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
SQL> alter session set events '10053 trace name context off';
會話已更改。
TRACE中
_optimizer_cost_model = io
Table Stats::
Table: T1 Alias: T1
#Rows: 10000 #Blks: 20 AvgRowLen: 3.00
***************************************
SINGLE TABLE ACCESS PATH
Table: T1 Alias: T1
Card: Original: 10000 Rounded: 10000 Computed: 10000.00 Non Adjusted: 10000.00
Access Path: TableScan
Cost: 4.00 Resp: 4.00 Degree: 0
Cost_io: 4.00 Cost_cpu: 0
Resp_io: 4.00 Resp_cpu: 0
Best:: AccessPath: TableScan
Cost: 4.00 Degree: 1 Resp: 4.00 Card: 10000.00 Bytes: 0
這個4 COST=(HWM下block/adjusted dbf_mbrc)得出
ADJUSTER_MBRC=blocs/cost=20/4=5(大約 應該反覆測試) 一般就是這麼推出的(9I 和10G 不一樣 )
db_file_multiblock_read_count Cost Adjusted dbf_mbrc(這個表來自COST-BASED ORACLE)9I條件下
4 2,396 4.17
8 1,518 6.59
16 962 10.40
32 610 16.39
64 387 25.84
128 245 40.82
與db_file_multiblock_read_count 大小有關係
Block Size Cost of 10,000 Block Scans Adjusted dbf_mbrc Cost for 80MB Scan(這個表來自COST-BASED ORACLE)9I條件下
2KB 611 16.39 2,439
4KB 963 10.40 1,925
8KB 1,519 6.59 1,519
8KB ASSM 1,540 n/a 1,540
16KB 2,397 4.17 1,199
塊大小的影響
SQL> create table t2 (a int);
表已建立。
SQL> ed
已寫入 file afiedt.buf
1 declare
2 begin
3 for i in 1..5000 loop
4 insert into t2 values(i);
5 end loop;
6* end;
SQL> /
PL/SQL 過程已成功完成。
SQL> commit;
提交完成。
SQL> exec dbms_stats.gather_table_stats('SYS','T2');
PL/SQL 過程已成功完成。
SQL> select blocks from user_tables where table_name='T2';
BLOCKS
----------
12
SQL> set autotrace traceonly
SQL> alter session set events '10053 trace name context forever';
會話已更改。
SQL> select * from t2;
已選擇5000行。
執行計劃
----------------------------------------------------------
Plan hash value: 1513984157
----------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
----------------------------------------------------------
| 0 | SELECT STATEMENT | | 5000 | 15000 | 3 |
| 1 | TABLE ACCESS FULL| T2 | 5000 | 15000 | 3 |
----------------------------------------------------------
Note
-----
- cpu costing is off (consider enabling it)~~~~~~~關閉
統計資訊
----------------------------------------------------------
135 recursive calls
0 db block gets
362 consistent gets
0 physical reads
0 redo size
88572 bytes sent via SQL*Net to client
4048 bytes received via SQL*Net from client
335 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
5000 rows processed
SQL> select ceil(12/5)from dual;
CEIL(12/5)
----------
3
TRACE中結果
_optimizer_cost_model = io
SINGLE TABLE ACCESS PATH
Table: T2 Alias: T2
Card: Original: 5000 Rounded: 5000 Computed: 5000.00 Non Adjusted: 5000.00
Access Path: TableScan
Cost: 3.00 Resp: 3.00 Degree: 0
Cost_io: 3.00 Cost_cpu: 0
Resp_io: 3.00 Resp_cpu: 0
Best:: AccessPath: TableScan
Cost: 3.00 Degree: 1 Resp: 3.00 Card: 5000.00 Bytes: 0
**********************關於 收集系統統計
SQL> alter system set "_optimizer_cost_model"=choose
2 ;
系統已更改。
SQL> exec dbms_stats.gather_system_stats('START');
PL/SQL 過程已成功完成。
SQL> select count(*) from t1;
COUNT(*)
----------
10000
SQL> select count(*) from t2;
COUNT(*)
----------
5000
SQL> select count(*) from dba_users;
COUNT(*)
----------
28
SQL> select count(*) from dba_tables;
COUNT(*)
----------
1582
SQL> exec dbms_stats.gather_system_stats('STop');~~~~~~~~~~~~~~~~執行一陣收集資訊
PL/SQL 過程已成功完成。
SQL> /
PNAME PVAL1
------------------------------ ----------
CPUSPEED 743
CPUSPEEDNW 484.974958
IOSEEKTIM 10
IOTFRSPEED 4096
MAXTHR 2048
MBRC 4
MREADTIM 6.364
SLAVETHR
SREADTIM 5.769
已選擇9行。
SQL> set autotrace traceonly
SQL> alter session set events '10053 trace name context forever';
會話已更改。
SQL> select * from t1;
已選擇10000行。
執行計劃
----------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10000 | 30000 | 7 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T1 | 10000 | 30000 | 7 (0)| 00:00:01 |
--------------------------------------------------------------------------
統計資訊
----------------------------------------------------------
1 recursive calls
0 db block gets
690 consistent gets
0 physical reads
0 redo size
176812 bytes sent via SQL*Net to client
7711 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
Cost = (
#SRds +
#MRds * mreadtim / sreadtim +**************io cost
#CPUCycles / (cpuspeed * sreadtim)********cpu cost
COST=20/4*6.364/5.796
SQL> select ceil(20/4*6.364/5.769) from dual;
CEIL(20/4*6.364/5.769)
----------------------
6
另外 table _table_scan_cost_plus_one 為TRUCE 所以 IO COST =7
TRACE中
_table_scan_cost_plus_one = true
SYSTEM STATISTICS INFORMATION
*****************************
Using WORKLOAD Stats
CPUSPEED: 743 millions instructions/sec
SREADTIM: 6 milliseconds
MREADTIM: 6 millisecons
MBRC: 4.000000 blocks
MAXTHR: 2048 bytes/sec
SLAVETHR: -1 bytes/sec
***************************************
BASE STATISTICAL INFORMATION
***********************
Table Stats::
Table: T1 Alias: T1
#Rows: 10000 #Blks: 20 AvgRowLen: 3.00
***************************************
SINGLE TABLE ACCESS PATH
Table: T1 Alias: T1
Card: Original: 10000 Rounded: 10000 Computed: 10000.00 Non Adjusted: 10000.00
Access Path: TableScan
Cost: 7.38 Resp: 7.38 Degree: 0
Cost_io: 7.00 Cost_cpu: 1642429
Resp_io: 7.00 Resp_cpu: 1642429
Best:: AccessPath: TableScan
Cost: 7.38 Degree: 1 Resp: 7.38 Card: 10000.00 Bytes: 0
這是採用收集系統統計資訊之後COST 有所上升
cpu部分計算=1642429/(743*5769)=0.38 將CPU_COST 換算成IO 單位
? CPUSPEED = 743MHZ
? sreadtim = 5769 milliseconds = 5769 microseconds (standardizing units of time)
? #CPUCycles (called cpu_cost in the plan_table) = 1642429
#CPUCycles / (cpuspeed * sreadtim)
SQL> select 1642429/(743*5769) from dual;
1642429/(743*5769)
------------------
.383175076
另外可以自己設定系統統計資訊
delete aux_stats$
execute
dbms_stats.set_system_stats('CPUSPEED',500);
dbms_stats.set_system_stats('SREADTIM',5.0);
dbms_stats.set_system_stats('MREADTIM',30.0);
dbms_stats.set_system_stats('MBRC',12);
小結:oracle 10g _optimizer_cost_model CHOOSE模式 基本就是使用CPU_COSTING計算,所以需要系統統計資訊 沒有時候用DEFALUT的,不再用8I 那樣傳統的COST計算(只簡單的計
算IO)
來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/12020513/viewspace-613616/，如需轉載，請註明出處，否則將追究法律責任。
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