[20190823]關於CPU成本計算2.txt--//前幾天探究CPU cost時遇到的問題,獲取行成本時我的測試查詢結果出現跳躍，不知道為什麼，感覺有點奇怪，分析看看。--//ITPUB原始鏈接已經不存在，我的日記本還有記錄，現在想想當時的記錄思路很亂，不過這些都是猜測的過程，以前思路混亂也是正 ...
[20190823]關於CPU成本計算2.txt
--//前幾天探究CPU cost時遇到的問題,獲取行成本時我的測試查詢結果出現跳躍，不知道為什麼，感覺有點奇怪，分析看看。
--//ITPUB原始鏈接已經不存在，我的日記本還有記錄，現在想想當時的記錄思路很亂，不過這些都是猜測的過程，以前思路混亂也是正常的。
--//順便做一些必要補充。
1.環境:
SCOTT@test01p> @ ver1
PORT_STRING VERSION BANNER CON_ID
-------------------- ---------- ---------------------------------------------------------------------------- ------
IBMPC/WIN_NT64-9.1.0 12.2.0.1.0 Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production 0
2.測試:
SCOTT@test01p> create table t as select rownum a1 , rownum a2 ,rownum a3 from dual connect by level<=100 ;
Table created.
--//分析略.
select 'explain plan set statement_id='''||lpad(rownum,3,'0')||''''||' for select 1 from t where rownum<='||rownum||';' c80 from t;
--//把以上的輸出保存一個文件執行,然後執行如下:
select STATEMENT_ID,CPU_COST,lead(cpu_cost ) over ( order by STATEMENT_ID ) N1,lead(cpu_cost ) over ( order by STATEMENT_ID )- cpu_cost N2 from (
select STATEMENT_ID,OPERATION, OPTIONS, COST, CPU_COST, IO_COST, TIME from plan_table where options='FULL');
STATEMENT_ CPU_COST N1 N2
---------- -------- ----- ----
001 7271 7421 150
002 7421 7571 150
003 7571 7721 150
004 7721 7871 150
005 7871 8021 150
006 8021 8321 300
007 8321 8321 0
008 8321 8471 150
009 8471 8621 150
010 8621 8771 150
011 8771 8921 150
012 8921 9071 150
013 9071 9371 300
014 9371 9371 0
015 9371 9521 150
016 9521 9671 150
017 9671 9821 150
018 9821 9971 150
019 9971 10121 150
020 10121 10271 150
021 10271 10421 150
022 10421 10571 150
023 10571 10721 150
024 10721 10871 150
025 10871 18143 7272
026 18143 18293 150
027 18293 18593 300
028 18593 18593 0
029 18593 18743 150
030 18743 18893 150
031 18893 19043 150
032 19043 19193 150
033 19193 19343 150
034 19343 19493 150
035 19493 19643 150
036 19643 19793 150
037 19793 19943 150
038 19943 20093 150
039 20093 20243 150
040 20243 20393 150
041 20393 20543 150
042 20543 20693 150
043 20693 20843 150
044 20843 20993 150
045 20993 21143 150
046 21143 21293 150
047 21293 21443 150
048 21443 21593 150
049 21593 21743 150
050 21743 29014 7271
051 29014 29164 150
052 29164 29314 150
053 29314 29464 150
054 29464 29914 450
055 29914 29914 0
056 29914 29914 0
057 29914 30064 150
058 30064 30214 150
059 30214 30364 150
060 30364 30514 150
061 30514 30664 150
062 30664 30814 150
063 30814 30964 150
064 30964 31114 150
065 31114 31264 150
066 31264 31414 150
067 31414 31564 150
068 31564 31714 150
069 31714 31864 150
070 31864 32014 150
071 32014 32164 150
072 32164 32314 150
073 32314 32464 150
074 32464 32614 150
075 32614 39886 7272
076 39886 40036 150
077 40036 40186 150
078 40186 40336 150
079 40336 40486 150
080 40486 40636 150
081 40636 40786 150
082 40786 40936 150
083 40936 41086 150
084 41086 41236 150
085 41236 41386 150
086 41386 41536 150
087 41536 41686 150
088 41686 41836 150
089 41836 41986 150
090 41986 42136 150
091 42136 42286 150
092 42286 42436 150
093 42436 42586 150
094 42586 42736 150
095 42736 42886 150
096 42886 43036 150
097 43036 43186 150
098 43186 43486 300
099 43486 43486 0
100 43486
100 rows selected.
--//大於7271的部分,我前面已經解析.
--//在STATEMENT_ID=025,050,075,N2分別是7272,7271,7272.說明在statement_id=026,051,076多訪問1塊。
--//可以這麼理解表T占4blocks,共100行，平均下來每塊25行。這樣當查詢等於rownum<=26,51,76時出現多訪問1塊的情況。
--//剩下就是為什麼查詢條件rownum<=55,rownum<=56,rownum<=57時CPU_COST一樣的,不好理解。N2出現跳躍的情況呢?
3.繼續探究:
SCOTT@test01p> set feedback only
SCOTT@test01p> select 1 from t where rownum<=55 ;
1
----------
55 rows selected.
SCOTT@test01p> set feedback 6
SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID g2r21fyyf3y90, child number 1
-------------------------------------
select 1 from t where rownum<=55
Plan hash value: 508354683
-------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 3 (100)| | 55 |00:00:00.01 | 6 |
|* 1 | COUNT STOPKEY | | 1 | | | | 55 |00:00:00.01 | 6 |
| 2 | TABLE ACCESS FULL| T | 1 | 57 | 3 (0)| 00:00:01 | 55 |00:00:00.01 | 6 |
-------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
2 - SEL$1 / T@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<=55)
--//註意看E-Rows = 57.噢!這樣明白為什麼出現跳躍.是oracle估計選擇率的演算法非常特別造成這樣的情況.
--//感覺oracle這樣條件演算法有點奇怪,什麼可能查詢條件rownum<=55,E-Rows = 57呢?
--//看看select 1 from t where rownum<=7的執行計劃也可以驗證:
SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID a8yj08mysamg1, child number 0
-------------------------------------
select 1 from t where rownum<=7
Plan hash value: 508354683
-------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 2 (100)| | 7 |00:00:00.01 | 6 |
|* 1 | COUNT STOPKEY | | 1 | | | | 7 |00:00:00.01 | 6 |
| 2 | TABLE ACCESS FULL| T | 1 | 8 | 2 (0)| 00:00:01 | 7 |00:00:00.01 | 6 |
-------------------------------------------------------------------------------------------------------------
--//E-ROWS=8.這樣就明白為什麼我當時的演算法每行成本出現跳躍的情況.其它大家可以自行驗證.
4.選擇率如何計算呢?
--//rownun<=N,這樣的查詢我看了<基於成本的Oracle優化法則>,也沒有這方面的內容.
--//我試圖按照區間的演算法不對.
--// Selectivity = (limit – low_value) / (high_value – low_value) + 1/num_distinct
--//(55-1)/(100-1)+1/100 = .55545454545454545454 , 不對,rownum虛擬列,這樣的查詢條件選擇率如何確定不知道.
SCOTT@test01p> set feedback only
SCOTT@test01p> select 1 from t where a1<=55 ;
1
----------
55 rows selected.
SCOTT@test01p> set feedback 6
SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 4vmjyzbu16y74, child number 0
-------------------------------------
select 1 from t where a1<=55
Plan hash value: 1601196873
--------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 3 (100)| | 55 |00:00:00.01 | 10 |
|* 1 | TABLE ACCESS FULL| T | 1 | 56 | 168 | 3 (0)| 00:00:01 | 55 |00:00:00.01 | 10 |
--------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / T@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("A1"<=55)
--//如果查詢select 1 from t where a1<=55;E-Rows=56,按照上面公式Selectivity = (limit – low_value) / (high_value – low_value) + 1/num_distinct
--//計算的結果是正確的.
--//僅僅知道為什麼出現上面的情況,不知道條件rownum<=N的選擇率計算公式.
--//如果加大NUMROWS=> 1000,就不會出現前面的情況.
SCOTT@test01p> exec dbms_stats.set_table_stats(ownname=> NULL,TABNAME=>'T',NUMROWS=> 1000);
PL/SQL procedure successfully completed.
SCOTT@test01p> alter system flush shared_pool;
System altered.
--//select 'explain plan set statement_id='''||lpad(rownum,3,'0')||''''||' for select 1 from t where rownum<='||rownum||';' c80 from t;
SCOTT@test01p> SELECT *
FROM (SELECT STATEMENT_ID,CPU_COST,lead(cpu_cost )
OVER ( ORDER BY STATEMENT_ID ) N1,lead(cpu_cost )
OVER ( ORDER BY STATEMENT_ID )- cpu_cost N2
FROM ( SELECT STATEMENT_ID,
OPERATION, OPTIONS, COST, CPU_COST, IO_COST,
TIME FROM plan_table WHERE options = 'FULL') )
WHERE N2 <> 150;
no rows selected
