国产99久久精品_欧美日本韩国一区二区_激情小说综合网_欧美一级二级视频_午夜av电影_日本久久精品视频

最新文章專題視頻專題問答1問答10問答100問答1000問答2000關鍵字專題1關鍵字專題50關鍵字專題500關鍵字專題1500TAG最新視頻文章推薦1 推薦3 推薦5 推薦7 推薦9 推薦11 推薦13 推薦15 推薦17 推薦19 推薦21 推薦23 推薦25 推薦27 推薦29 推薦31 推薦33 推薦35 推薦37視頻文章20視頻文章30視頻文章40視頻文章50視頻文章60 視頻文章70視頻文章80視頻文章90視頻文章100視頻文章120視頻文章140 視頻2關鍵字專題關鍵字專題tag2tag3文章專題文章專題2文章索引1文章索引2文章索引3文章索引4文章索引5123456789101112131415文章專題3
問答文章1 問答文章501 問答文章1001 問答文章1501 問答文章2001 問答文章2501 問答文章3001 問答文章3501 問答文章4001 問答文章4501 問答文章5001 問答文章5501 問答文章6001 問答文章6501 問答文章7001 問答文章7501 問答文章8001 問答文章8501 問答文章9001 問答文章9501
當前位置： 首頁 - 科技 - 知識百科 - 正文

示例演示直方圖的重要性

來源：懂視網 責編：小采 時間：2020-11-09 07:33:27
文檔

示例演示直方圖的重要性

示例演示直方圖的重要性:1. 示例說明直方圖的作用。 初始化數據 dexter@STARTREK select count(*) fromall_objects ; COUNT(*) ---------- 72642 dexter@STARTREK create table tuning4_tabnologging as select * from all_obje
推薦度：
導讀示例演示直方圖的重要性:1. 示例說明直方圖的作用。 初始化數據 dexter@STARTREK select count(*) fromall_objects ; COUNT(*) ---------- 72642 dexter@STARTREK create table tuning4_tabnologging as select * from all_obje

1. 示例說明直方圖的作用。 初始化數據 dexter@STARTREK select count(*) fromall_objects ; COUNT(*) ---------- 72642 dexter@STARTREK create table tuning4_tabnologging as select * from all_objects ; Table created. dexter@STARTREK select count(*

1. 示例說明直方圖的作用。

初始化數據

dexter@STARTREK> select count(*) fromall_objects ;

COUNT(*)

----------

72642

dexter@STARTREK> create table tuning4_tabnologging as select * from all_objects ;

Table created.

dexter@STARTREK> select count(*) fromall_objects ;

COUNT(*)

----------

72643

dexter@STARTREK> create indexidx_tuning4_tab_owner on tuning4_tab (owner) ;

Index created.

dexter@STARTREK> @gather_tab

Enter value for tbname: tuning4_tab

PL/SQL procedure successfully completed.

在這里碰到了一個小問題，因為數據的傾斜比較嚴重，而且oracle數據庫在執行gather_table_stats的時候沒有收集owner列的統計信息，這里優化器選擇了錯誤的執行計劃。下面記錄了完整的處理過程。

出現錯誤的執行計劃

dexter@STARTREK> select* from tuning4_Tab where owner='SYS' or owner='PUBLIC' ;

59253 rows selected.

Execution Plan

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

Plan hash value: 989038285

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

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

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

| 0 | SELECT STATEMENT | | 4687 | 443K| 139 (0)| 00:00:01 |

| 1 | INLIST ITERATOR | | | | | |

| 2 | TABLE ACCESS BY INDEX ROWID| TUNING4_TAB | 4687 | 443K| 139 (0)| 00:00:01 |

|* 3 | INDEX RANGE SCAN |IDX_TUNING4_TAB_OWNER | 4687 | | 13 (0)| 00:00:01 |

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

Predicate Information(identified by operation id):

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

3 - access("OWNER"='PUBLIC' OR"OWNER"='SYS')

Statistics

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

1 recursive calls

0 db block gets

9581 consistent gets

0 physical reads

0 redo size

6805858 bytes sent via SQL*Net to client

43970 bytes received via SQL*Net from client

3952 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

59253 rows processed

下面的輸出可以看到，其實使用全表掃描的效率要高于indexrange scan。

dexter@STARTREK> select /*+full(tuning4_tab)*/ *from tuning4_Tab where owner='SYS' or owner='PUBLIC' ;

59253 rows selected.

Execution Plan

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

Plan hash value: 1641193091

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

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

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

| 0 |SELECT STATEMENT | | 4687 | 443K| 290 (1)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| TUNING4_TAB | 4687 | 443K| 290 (1)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 -filter("OWNER"='PUBLIC' OR "OWNER"='SYS')

Statistics

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

1 recursive calls

0 db block gets

4927 consistent gets

0 physical reads

0 redo size

3035580 bytes sent via SQL*Net toclient

43970 bytes received via SQL*Netfrom client

3952 SQL*Net roundtrips to/fromclient

0 sorts (memory)

0 sorts (disk)

59253 rows processed

dexter@STARTREK>

trace 一下

dexter@STARTREK> alter session settracefile_identifier=histogram ;

Session altered.

dexter@STARTREK> alter session set events '10053trace name context forever ,level 12';

Session altered.

dexter@STARTREK> select * from tuning4_Tab whereowner='SYS' or owner='PUBLIC' ;

59253 rows selected.

Execution Plan

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

Plan hash value: 989038285

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

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

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

| 0 | SELECT STATEMENT | | 4687 | 443K| 139 (0)| 00:00:01 |

| 1 | INLIST ITERATOR | | | | | |

| 2 | TABLE ACCESS BY INDEX ROWID| TUNING4_TAB | 4687 | 443K| 139 (0)| 00:00:01 |

|* 3 | INDEX RANGE SCAN |IDX_TUNING4_TAB_OWNER | 4687 | | 13 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

3 -access("OWNER"='PUBLIC' OR "OWNER"='SYS')

Statistics

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

0 recursive calls

0 db block gets

9581 consistent gets

0 physical reads

0 redo size

6805858 bytes sent via SQL*Net toclient

43970 bytes received via SQL*Netfrom client

3952 SQL*Net roundtrips to/fromclient

0 sorts (memory)

0 sorts (disk)

59253 rows processed

dexter@STARTREK> alter session set events '10053trace name context off ';

Session altered.

dexter@STARTREK>

從10053中看到

Access path analysis for TUNING4_TAB

***************************************

SINGLE TABLE ACCESS PATH

SingleTable Cardinality Estimation for TUNING4_TAB[TUNING4_TAB]

Column(#1): OWNER(

AvgLen: 6NDV: 31 Nulls: 0 Density: 0.032258

Table:TUNING4_TAB Alias: TUNING4_TAB

Card:Original: 72643.000000 Rounded: 4687 Computed: 4686.65 Non Adjusted: 4686.65

Rounded:

4687

實際：

59253

明顯是由于統計信息不準確造成的。我們看一下它的直方圖信息。

其實從執行計劃

| 0 | SELECTSTATEMENT | | 4687 | 443K| 139 (0)| 00:00:01 |

也可以看到它的統計信息不準確。

確定問題根源

dexter@STARTREK> select* from user_tab_histograms where table_name='TUNING4_TAB' andcolumn_name='OWNER' ;

TABLE_NAME COLUMN_NAME ENDPOINT_NUMBERENDPOINT_VALUE ENDPOINT_ACTUAL_VALUE

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

TUNING4_TAB OWNER 0 3.3913E+35

TUNING4_TAB OWNER 1 4.5831E+35

dexter@STARTREK> select table_name , column_name, histogram from user_tab_col_statistics where table_name='TUNING4_TAB' ;

TABLE_NAME COLUMN_NAME HISTOGRAM

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

TUNING4_TAB OWNER NONE

TUNING4_TAB OBJECT_NAME NONE

TUNING4_TAB SUBOBJECT_NAME NONE

TUNING4_TAB OBJECT_ID NONE

TUNING4_TAB DATA_OBJECT_ID NONE

TUNING4_TAB OBJECT_TYPE NONE

TUNING4_TAB CREATED NONE

TUNING4_TAB LAST_DDL_TIME NONE

TUNING4_TAB TIMESTAMP NONE

TUNING4_TAB STATUS NONE

TUNING4_TAB TEMPORARY NONE

TUNING4_TAB GENERATED NONE

TUNING4_TAB SECONDARY NONE

TUNING4_TAB NAMESPACE NONE

TUNING4_TAB EDITION_NAME NONE

15 rows selected.

沒有直方圖信息。

我們來直接查看表中數據的分布情況

dexter@STARTREK> select owner,count(*) fromtuning4_Tab group by owner order by 2 ;

OWNER COUNT(*)

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

OWBSYS 2

APPQOSSYS 5

SCOTT 6

SI_INFORMTN_SCHEMA 8

OUTLN 8

ORACLE_OCM 8

BI 8

ORDPLUGINS 10

PM 10

FLOWS_FILES 11

OWBSYS_AUDIT 12

DEXTER 25

APEX 33

HR 35

IX 48

DBSNMP 57

OE 112

ORDDATA 239

SH 299

EXFSYS 308

WMSYS 312

CTXSYS 384

SYSTEM 516

XDB 517

OLAPSYS 717

MDSYS 1545

APEX_030200 2251

ORDSYS 2512

SYSMAN 3392

PUBLIC 28027

SYS 31226

31 rows selected.

傾斜很嚴重，并且因為gather_table_stats的時候默認沒有收集直方圖信息，導致優化器沒有選擇正確的執行計劃，我們來收集一下它的直方圖。

默認為FOR ALL COLUMNS SIZEAUTO沒有收集直方圖。

dexter@STARTREK> selectdbms_stats.get_param('METHOD_OPT') from dual ;

DBMS_STATS.GET_PARAM('METHOD_OPT')

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

FOR ALL COLUMNS SIZE AUTO

收集列的直方圖信息

dexter@STARTREK> execdbms_stats.gather_table_stats(user,'tuning4_tab',cascade=>true,method_opt=>'FORALL columns size skewonly') ;

PL/SQL procedure successfully completed.

dexter@STARTREK> select table_name , column_name, histogram from user_tab_col_statistics where table_name='TUNING4_TAB' ;

TABLE_NAME COLUMN_NAME HISTOGRAM

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

TUNING4_TAB OWNER FREQUENCY

TUNING4_TAB OBJECT_NAME HEIGHT BALANCED

TUNING4_TAB SUBOBJECT_NAME FREQUENCY

TUNING4_TAB OBJECT_ID NONE

TUNING4_TAB DATA_OBJECT_ID HEIGHT BALANCED

TUNING4_TAB OBJECT_TYPE FREQUENCY

TUNING4_TAB CREATED HEIGHT BALANCED

TUNING4_TAB LAST_DDL_TIME HEIGHT BALANCED

TUNING4_TAB TIMESTAMP HEIGHT BALANCED

TUNING4_TAB STATUS FREQUENCY

TUNING4_TAB TEMPORARY FREQUENCY

TUNING4_TAB GENERATED FREQUENCY

TUNING4_TAB SECONDARY FREQUENCY

TUNING4_TAB NAMESPACE FREQUENCY

TUNING4_TAB EDITION_NAME NONE

15 rows selected.

owner為頻率直方圖，比較正確。

dexter@STARTREK> select * from user_tab_histograms wheretable_name='TUNING4_TAB' and column_name='OWNER' ;

TABLE_NAME COLUMN_NAME ENDPOINT_NUMBERENDPOINT_VALUE ENDPOINT_ACTUAL_VALUE

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

TUNING4_TAB OWNER 1 3.3913E+35

TUNING4_TAB OWNER 179 3.3913E+35

TUNING4_TAB OWNER 206 3.4959E+35

TUNING4_TAB OWNER 212 3.5442E+35

TUNING4_TAB OWNER 215 3.5448E+35

TUNING4_TAB OWNER 244 3.6006E+35

TUNING4_TAB OWNER 246 3.7551E+35

TUNING4_TAB OWNER 249 3.8082E+35

TUNING4_TAB OWNER 370 4.0119E+35

TUNING4_TAB OWNER 383 4.1159E+35

TUNING4_TAB OWNER 422 4.1174E+35

TUNING4_TAB OWNER 423 4.1186E+35

TUNING4_TAB OWNER 436 4.1186E+35

TUNING4_TAB OWNER 636 4.1186E+35

TUNING4_TAB OWNER 2824 4.1711E+35

TUNING4_TAB OWNER 2855 4.3242E+35

TUNING4_TAB OWNER 5199 4.3277E+35

TUNING4_TAB OWNER 5455 4.3277E+35

TUNING4_TAB OWNER 5500 4.3277E+35

TUNING4_TAB OWNER 5525 4.5330E+35

TUNING4_TAB OWNER 5567 4.5831E+35

21 rows selected.

再次測試

已經收集好了直方圖，我們再來看一下執行計劃以及10053事件。

set autotrace traceonly

alter session set tracefile_identifier=histogram ;

alter session set events '10053 trace name contextforever ,level 12';

select * from tuning4_Tab where owner='SYS' orowner='PUBLIC' ;

alter session set events '10053 trace name contextoff ';

已經選擇了正確、最優的執行計劃。

dexter@STARTREK> set autotrace traceonly

dexter@STARTREK> alter session settracefile_identifier=histogram ;

Session altered.

dexter@STARTREK> alter session set events '10053trace name context forever ,level 12';

Session altered.

dexter@STARTREK> select * from tuning4_Tab whereowner='SYS' or owner='PUBLIC' ;

59253 rows selected.

Execution Plan

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

Plan hash value: 1641193091

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

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

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

| 0 |SELECT STATEMENT | | 59137 | 5601K| 291 (1)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| TUNING4_TAB | 59137 | 5601K| 291 (1)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 -filter("OWNER"='PUBLIC' OR "OWNER"='SYS')

Statistics

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

0 recursive calls

0 db block gets

4927 consistent gets

0 physical reads

0 redo size

3035580 bytes sent via SQL*Net toclient

43970 bytes received via SQL*Netfrom client

3952 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

59253 rows processed

dexter@STARTREK> alter session set events '10053trace name context off ';

Session altered.

dexter@STARTREK> exit

SINGLE TABLE ACCESS PATH

SingleTable Cardinality Estimation for TUNING4_TAB[TUNING4_TAB]

Column(#1):

NewDensity:0.000090, OldDensity:0.000007 BktCnt:5567, PopBktCnt:5565,PopValCnt:19, NDV:31

Column(#1): OWNER(

AvgLen: 6NDV: 31 Nulls: 0 Density: 0.000090

Histogram: Freq #Bkts: 21 UncompBkts: 5567 EndPtVals: 21

Table:TUNING4_TAB Alias: TUNING4_TAB

Card:Original: 72643.000000 Rounded: 59137 Computed: 59137.43 Non Adjusted: 59137.43

Rounded: 59137

比較正確了。也選擇了全表掃描作為最佳的accesspath。

聲明：本網頁內容旨在傳播知識，若有侵權等問題請及時與本網聯系，我們將在第一時間刪除處理。TEL:177 7030 7066 E-MAIL:11247931@qq.com

文檔

示例演示直方圖的重要性

示例演示直方圖的重要性:1. 示例說明直方圖的作用。 初始化數據 dexter@STARTREK select count(*) fromall_objects ; COUNT(*) ---------- 72642 dexter@STARTREK create table tuning4_tabnologging as select * from all_obje
推薦度：
標簽： 作用 直方圖 演示
  • 熱門焦點

最新推薦

猜你喜歡

熱門推薦

專題
Top
主站蜘蛛池模板： 欧美午夜网 | 免费一级毛片视频 | 国产区最新 | er久99久热只有精品国产 | 亚洲一区二区在线 | 亚洲视频在线观看网站 | 国产69久久精品成人看小说 | 国产在视频 | 日韩影音| 亚欧洲精品在线视频免费观看 | 亚洲图片欧美日韩 | 国产精品第1页在线观看 | 久久精品一区二区国产 | 欧美成人看片一区二区三区尤物 | 免费观看a毛片一区二区不卡 | 欧美色图一区 | 欧美日韩高清完整版在线观看免费 | 国产精品视频第一区二区三区 | 国产精品第十页 | 国产亚洲精品a在线观看app | 日本大黄在线观看 | 明星国产欧美日韩在线观看 | 一区在线观看 | 欧美中文娱乐网 | 欧美第1页 | 99久久免费国产精精品 | 岛国一区| 国产高清在线精品一区二区三区 | 草逼电影 | 精品1区2区3区 | 亚洲三级电影网站 | 久久精品视频一区二区三区 | 国产日韩在线看 | 国产在线视频在线 | 欧美日韩亚洲v在线观看 | www.日韩视频| 国产一级高清免费观看 | 日韩在线不卡 | 欧美 第一页 | 精品国产免费人成在线观看 | 日韩亚洲欧美一区 |