[20170603]12c Top Frequency histogram.txt

lfree發表於2017-06-03
[20170603]12c Top Frequency histogram.txt

--//個人對直方圖瞭解很少,以前2種直方圖型別對於目前的許多應用來講已經足夠,或者講遇到的問題很少.
--//抽一點點時間,簡單探究12c Top Frequency histogram.

--//以前的頻率直方圖Frequency histogram,受限bucket(桶的大小),如果有255個不同值,oracle分析後不會建立頻率直方圖,而是建立高
--//度直方圖.這樣的情況會導致一些流行值的統計在顯示執行計劃時差距很大.而12c引入了Top Frequency histogram,注意這裡的top,
--//我的理解就是流行值(popular),也就是這樣建立的直方圖僅僅包括popular,其他non-popular不考慮,這樣在sql語句的查詢這些
--//popular時,顯示的統計資訊相對準確,從而有利於oracle選擇正確的執行計劃.

--//以下是我的學習筆記,也許會存在許多錯誤,僅僅做一個記錄.我也看了許多別人的blog.^_^.而且我目前的環境只有12.0.0.1(版本太
--//低).

1.環境:
SCOTT@test01p> @ ver1
PORT_STRING                    VERSION        BANNER                                                                               CON_ID
------------------------------ -------------- -------------------------------------------------------------------------------- ----------
IBMPC/WIN_NT64-9.1.0           12.1.0.1.0     Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production              0

--//如果要建立Top Frequency histogram必須要滿足幾個條件:
--//連結 raajeshwaran.blogspot.co.id/2016/06/top-frequency-histogram-in-12c.html

The database creates a Top frequency histogram, when the following criteria are met.

NDV is greater than n, where n is the requested number of buckets (default 254)
The percentage of rows occupied by Top-frequent values is greater than or equal to the threshold p where p is (1-(1/n)*100).
The estimate_percent parameter in dbms_stats gathering procedure should be auto_sample_size (set to default)

--//翻譯過來NDV(也就是欄位的不同值)大於N(指bucket的數量).
--//流行值(popular)在Top-frequent中合計數量/總計數量之比要大於(1-(1/n)*100).如果建立10個桶,這樣流行值的總計必須在90%以上

2.首先驗證(1-(1/n)*100)比值是否正確:
SCOTT@test01p> create table t as select * from dba_objects;
Table created.

select column_name,num_distinct,density,histogram,SAMPLE_SIZE
  from user_tab_col_statistics
  where table_name ='T'
  and column_name ='OWNER';

COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       SAMPLE_SIZE
-------------------- ------------ ---------- --------------- -----------
OWNER                          32     .03125 NONE                  91695

--//12c ctas 建立統計資訊,但是不會建立直方圖.density 1/32=.03125.
SCOTT@test01p> select count(*) from t;
  COUNT(*)
----------
     91695

--//隨手寫的sql語句:

with a as (select distinct owner,count(*) over(partition by owner) n1 ,count(*) over () n2 from t order by 2 desc ),
b as (select owner,n1,n2,sum(n1) over (order by n1 desc) n3  from a order by n1 desc)
select rownum,owner,n1,n2,n3,round(n3/n2,5) x1,round(1-1/rownum,5) x2 from b;

ROWNUM OWNER                N1         N2         N3         X1         X2
------ ----------------- ----- ---------- ---------- ---------- ----------
     1 SYS               41942      91695      41942     .45741          0
     2 PUBLIC            37142      91695      79084     .86247         .5
     3 APEX_040200        3405      91695      82489      .8996     .66667
     4 ORDSYS             3157      91695      85646     .93403        .75
     5 MDSYS              1819      91695      87465     .95387         .8
     6 XDB                 985      91695      88450     .96461     .83333
     7 SYSTEM              641      91695      89091      .9716     .85714
     8 CTXSYS              405      91695      89496     .97602       .875
     9 WMSYS               387      91695      89883     .98024     .88889
    10 DVSYS               352      91695      90235     .98408         .9
    11 SH                  309      91695      90544     .98745     .90909
    12 ORDDATA             292      91695      90836     .99063     .91667
    13 LBACSYS             209      91695      91045     .99291     .92308
    14 OE                  142      91695      91187     .99446     .92857
    15 SCOTT                96      91695      91283     .99551     .93333
    16 GSMADMIN_INTERNAL    77      91695      91360     .99635      .9375
    17 IX                   58      91695      91418     .99698     .94118
    18 DBSNMP               55      91695      91473     .99758     .94444
    19 PM                   44      91695      91517     .99806     .94737
    20 HR                   35      91695      91552     .99844        .95
    21 OLAPSYS              25      91695      91577     .99871     .95238
    22 OJVMSYS              23      91695      91600     .99896     .95455
    23 DVF                  19      91695      91619     .99917     .95652
    24 FLOWS_FILES          13      91695      91632     .99931     .95833
    25 AUDSYS               12      91695      91644     .99944        .96
    26 ORDPLUGINS           10      91695      91664     .99966     .96154
    27 OUTLN                10      91695      91664     .99966     .96296
    28 BI                    8      91695      91688     .99992     .96429
    29 ORACLE_OCM            8      91695      91688     .99992     .96552
    30 SI_INFORMTN_SCHEM     8      91695      91688     .99992     .96667
    31 APPQOSSYS             5      91695      91693     .99998     .96774
    32 TEST                  2      91695      91695          1     .96875

--//如果加入條件where round(n3/n2,5) >round(1-1/rownum,5),全部輸出.也就是這樣如果桶小於32,大於1.建立的都是Top Frequency.

3.繼續測試:
D:\temp>cat a1.sql
cat a1.sql
exec  dbms_stats.gather_table_stats(ownname=>user,tabname=>'T',method_opt=>'for columns owner size &1');
select column_name,num_distinct,density,histogram,SAMPLE_SIZE from user_tab_col_statistics where table_name ='T' and column_name ='OWNER';

SCOTT@test01p> @ a1.sql 2
PL/SQL procedure successfully completed.

COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       SAMPLE_SIZE
-------------------- ------------ ---------- --------------- -----------
OWNER                          32     .03125 HYBRID                 5500

SCOTT@test01p> @ a1.sql 3
PL/SQL procedure successfully completed.

COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       SAMPLE_SIZE
-------------------- ------------ ---------- --------------- -----------
OWNER                          32 5.4529E-06 TOP-FREQUENCY         91695

SCOTT@test01p> @ a1.sql 4
PL/SQL procedure successfully completed.

COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       SAMPLE_SIZE
-------------------- ------------ ---------- --------------- -----------
OWNER                          32 5.4529E-06 TOP-FREQUENCY         91695

SCOTT@test01p> @ a1.sql 31
PL/SQL procedure successfully completed.
COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       SAMPLE_SIZE
-------------------- ------------ ---------- --------------- -----------
OWNER                          32 5.4529E-06 TOP-FREQUENCY         91695

SCOTT@test01p> @ a1.sql 32
PL/SQL procedure successfully completed.

COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       SAMPLE_SIZE
-------------------- ------------ ---------- --------------- -----------
OWNER                          32 5.4529E-06 FREQUENCY             91695

--//除了bucket=2,32建立的直方圖HYBRID,FREQUENCY外,建立的都是TOP-FREQUENCY.
--//以10個bucket為例.解方程式(90235-x)/(91695-x)=0.9 ,得到x=77095.也就是要減少77095.

--//delete t where owner='SYS' and rownum<=41000;
--//delete t where owner='PUBLIC' and rownum<=36095;

SCOTT@test01p> delete t where owner='SYS' and rownum<=41000;
41000 rows deleted.

SCOTT@test01p> delete t where owner='PUBLIC' and rownum<=36095;
36095 rows deleted.

SCOTT@test01p> commit ;
Commit complete.

with a as (select distinct owner,count(*) over(partition by owner) n1 ,count(*) over () n2 from t order by 2 desc ),
b as (select owner,n1,n2,sum(n1) over (order by n1 desc) n3  from a order by n1 desc)
select rownum,owner,n1,n2,n3,round(n3/n2,5) x1,round(1-1/rownum,5) x2 from b where rownum<=11;

ROWNUM OWNER         N1         N2         N3         X1         X2
------ ----------- ---- ---------- ---------- ---------- ----------
     1 APEX_040200 3405      14600       3405     .23322          0
     2 ORDSYS      3157      14600       6562     .44945         .5
     3 MDSYS       1819      14600       8381     .57404     .66667
     4 PUBLIC      1047      14600       9428     .64575        .75
     5 XDB          985      14600      10413     .71322         .8
     6 SYS          942      14600      11355     .77774     .83333
     7 SYSTEM       641      14600      11996     .82164     .85714
     8 CTXSYS       405      14600      12401     .84938       .875
     9 WMSYS        387      14600      12788     .87589     .88889
    10 DVSYS        352      14600      13140         .9         .9
    11 SH           309      14600      13449     .92116     .90909
11 rows selected.
--//backet=10,前面10個值佔90%.

SCOTT@test01p> @ a1 10
PL/SQL procedure successfully completed.
COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       SAMPLE_SIZE
-------------------- ------------ ---------- --------------- -----------
OWNER                          32 .000034247 TOP-FREQUENCY         14600

--//再減少1條記錄.
SCOTT@test01p> delete t where owner='SYS' and rownum<=1;
1 row deleted.

SCOTT@test01p> commit ;
Commit complete.

ROWNUM OWNER         N1         N2         N3         X1         X2
------ ----------- ---- ---------- ---------- ---------- ----------
     1 APEX_040200 3405      14599       3405     .23324          0
     2 ORDSYS      3157      14599       6562     .44948         .5
     3 MDSYS       1819      14599       8381     .57408     .66667
     4 PUBLIC      1047      14599       9428      .6458        .75
     5 XDB          985      14599      10413     .71327         .8
     6 SYS          941      14599      11354     .77772     .83333
     7 SYSTEM       641      14599      11995     .82163     .85714
     8 CTXSYS       405      14599      12400     .84937       .875
     9 WMSYS        387      14599      12787     .87588     .88889
    10 DVSYS        352      14599      13139     .89999         .9
    11 SH           309      14599      13448     .92116     .90909
11 rows selected.
--//現在前10佔.89999.

SCOTT@test01p> @ a1 10
PL/SQL procedure successfully completed.

COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       SAMPLE_SIZE
-------------------- ------------ ---------- --------------- -----------
OWNER                          32    .018378 HYBRID                14599

--//可以發現建立的直方圖不是TOP-FREQUENCY,而是HYBRID(混合型直方圖).

4.轉化成TOP-FREQUENCY.
SCOTT@test01p> insert into t  select * from dba_objects where owner='SYS' and rownum=1;
1 row created.

SCOTT@test01p> commit ;
Commit complete.

SCOTT@test01p> @ a1 10
PL/SQL procedure successfully completed.

COLUMN_NAME          NUM_DISTINCT    DENSITY HISTOGRAM       SAMPLE_SIZE
-------------------- ------------ ---------- --------------- -----------
OWNER                          32 .000034247 TOP-FREQUENCY         14600

--//DENSITY=1/SAMPLE_SIZE/2, 1/14600/2=.00003424657534246575正好符合.

5.現在看看執行計劃:
SCOTT@test01p> select count(*) from t where owner='DVSYS';
  COUNT(*)
----------
       352

SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  2at34f0zaqhzj, child number 0
-------------------------------------
select count(*) from t where owner='DVSYS'
Plan hash value: 2966233522
---------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
---------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |       |   428 (100)|          |      1 |00:00:00.01 |    1544 |
|   1 |  SORT AGGREGATE    |      |      1 |      1 |     8 |            |          |      1 |00:00:00.01 |    1544 |
|*  2 |   TABLE ACCESS FULL| T    |      1 |    352 |  2816 |   428   (0)| 00:00:01 |    352 |00:00:00.01 |    1544 |
---------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   2 - SEL$1 / T@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter("OWNER"='DVSYS')

--//可以發現e_rows=A-Rows.可以發現非常準確.
--//檢視直方圖資訊.

SCOTT@test01p> select endpoint_number,endpoint_actual_value from user_tab_histograms where table_name ='T' and column_name ='OWNER' order by 1;
ENDPOINT_NUMBER ENDPOINT_ACTUAL_VALU
--------------- --------------------
           3405 APEX_040200
           3810 CTXSYS
           4162 DVSYS
           5981 MDSYS
           9138 ORDSYS
          10185 PUBLIC
          11127 SYS
          11768 SYSTEM
          12155 WMSYS
          13140 XDB
10 rows selected.

--//4162-3810=352,可以發現正好符合.也就是popular值統計很正確.看看非popular值.

SCOTT@test01p> select count(*) from t where owner='DVSYS1';
  COUNT(*)
----------
         0

Plan hash value: 2966233522        
---------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
---------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |       |   428 (100)|          |      1 |00:00:00.01 |    1544 |
|   1 |  SORT AGGREGATE    |      |      1 |      1 |     8 |            |          |      1 |00:00:00.01 |    1544 |
|*  2 |   TABLE ACCESS FULL| T    |      1 |     66 |   528 |   428   (0)| 00:00:01 |      0 |00:00:00.01 |    1544 |
---------------------------------------------------------------------------------------------------------------------

SCOTT@test01p> select count(*) from t where owner='SCOTT';
  COUNT(*)
----------
        96

Plan hash value: 2966233522
---------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
---------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |       |   428 (100)|          |      1 |00:00:00.01 |    1544 |
|   1 |  SORT AGGREGATE    |      |      1 |      1 |     8 |            |          |      1 |00:00:00.01 |    1544 |
|*  2 |   TABLE ACCESS FULL| T    |      1 |     66 |   528 |   428   (0)| 00:00:01 |     96 |00:00:00.01 |    1544 |
---------------------------------------------------------------------------------------------------------------------

--//可以發現估計E-Rows如何計算的呢?

6.做10053跟蹤:
D:\tools\sqllaji>cat 10053x.sql
execute dbms_sqldiag.dump_trace(p_sql_id=>'&1',p_child_number=>&2,p_component=>'Compiler',p_file_id=>'&&1');

SCOTT@test01p> @ 10053x f31kz63ksu1tc 0
PL/SQL procedure successfully completed.


***************************************
SINGLE TABLE ACCESS PATH
  Single Table Cardinality Estimation for T[T]
SPD: Return code in qosdDSDirSetup: NOCTX, estType = TABLE
  Column (#1):
    NewDensity:0.004545, OldDensity:0.000034 BktCnt:13140.000000, PopBktCnt:13140.000000, PopValCnt:10, NDV:32
  Column (#1): OWNER(VARCHAR2)
    AvgLen: 8 NDV: 32 Nulls: 0 Density: 0.000000
    Histogram: Top-Freq  #Bkts: 13140  UncompBkts: 13140  EndPtVals: 10  ActualVal: yes
  Table: T  Alias: T
    Card: Original: 14600.000000  Rounded: 66  Computed: 66.36  Non Adjusted: 66.36
  Access Path: TableScan
    Cost:  428.36  Resp: 428.36  Degree: 0
      Cost_io: 428.00  Cost_cpu: 14122025
      Resp_io: 428.00  Resp_cpu: 14122025
  Best:: AccessPath: TableScan
         Cost: 428.36  Degree: 1  Resp: 428.36  Card: 66.36  Bytes: 0


    check parallelism for statement[<unnamed>]
kkfdtParallel: parallel is possible (no statement type restrictions)
    kkfdPaForcePrm: dop:1 ()
     use dictionary DOP(1) on table
kkfdPaPrm:- The table : 106380
kkfdPaPrm:DOP = 1 (computed from hint/dictionary/autodop)
kkfdiPaPrm: dop:1 serial(?)
***************************************

--//使用 NewDensity:0.004545.
 BktCnt:13140.000000, PopBktCnt:13140.000000 => 對應就是前10個流行值的總和.

--//非流行值的數量: 14600-13140=1460
--//非流行值的桶數量: 32-10=22
--//非流行值的數量/非流行值的桶數量 1460/22=66.36363636363636363636,四捨五入66,正好符合執行計劃的推斷.
--//NewDensity的計算 =1460/14600/22=.00454545454545454545,非常接近.



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