2. 程式人生 > >[20170612]FOR ALL COLUMNS SIZE repeat(11g).txt

[20170612]FOR ALL COLUMNS SIZE repeat(11g).txt

阿新 • • 發佈：2017-06-12
ets session ogr bject ber procedure glob 註意 com

[20170612]FOR ALL COLUMNS SIZE repeat(11g).txt

--//昨天看了https://jonathanlewis.wordpress.com/2017/06/01/histogram-upgrade-2/,提到了直方圖的問題,
--//特別是FOR ALL COLUMNS SIZE repeat引起的問題,在一些特殊情況要註意.

1.環境:
[email protected]> @ &r/ver1

PORT_STRING VERSION BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

create table t (id number,pad varchar2(20),flag varchar2(1));
insert into t select rownum,lpad(‘a‘,20,‘a‘) ,‘a‘ from dual connect by level<=2e4;
insert into t select rownum+2e4,lpad(‘d‘,20,‘d‘) ,‘d‘ from dual connect by level<=2;
commit ;

[email protected]

/* */> SELECT flag, COUNT(*) FROM t GROUP BY flag;
F COUNT(*)
- ----------
d 2
a 20000

[email protected]> exec dbms_stats.Gather_table_stats(ownname => user, tabname => ‘T‘,method_opt => ‘FOR ALL COLUMNS ‘);
PL/SQL procedure successfully completed.

select * from DBA_TAB_COL_STATISTICS where owner=user and table_name=‘T‘ and column_name=‘FLAG‘;

Record View
As of: 2017/6/12 9:35:03

OWNER: SCOTT
TABLE_NAME: T
COLUMN_NAME: FLAG
NUM_DISTINCT: 2
LOW_VALUE: 61
HIGH_VALUE: 64
DENSITY: 0.000024997500249975
NUM_NULLS: 0
NUM_BUCKETS: 2
LAST_ANALYZED: 2017/6/12 9:33:35
SAMPLE_SIZE: 20002
GLOBAL_STATS: YES
USER_STATS: NO
AVG_COL_LEN: 2
HISTOGRAM: FREQUENCY

--//建立了直方圖.

[email protected]
/* */> set numw 36
[email protected]> select * from USER_TAB_HISTOGRAMS where column_name=‘FLAG‘;
TABLE_NAME COLUMN_NAME ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ----------- --------------- ------------------------------------ ----------
T FLAG 20000 503652795277878000000000000000000000
T FLAG 20002 519229685853483000000000000000000000

--//如果應用模式發生變化,加入了2種值.

insert into t select rownum+2e4+2,lpad(‘b‘,20,‘b‘) ,‘b‘ from dual connect by level<=2e4;
insert into t select rownum+4e4+2,lpad(‘c‘,20,‘c‘) ,‘c‘ from dual connect by level<=2;
commit ;

[email protected]
/* */> exec dbms_stats.Gather_table_stats(ownname => user, tabname => ‘T‘,method_opt => ‘FOR ALL COLUMNS size repeat‘,no_invalidate=> false);
PL/SQL procedure successfully completed.

select * from DBA_TAB_COL_STATISTICS where owner=user and table_name=‘T‘ and column_name=‘FLAG‘;

Record View
As of: 2017/6/12 9:38:59

OWNER: SCOTT
TABLE_NAME: T
COLUMN_NAME: FLAG
NUM_DISTINCT: 4
LOW_VALUE: 61
HIGH_VALUE: 64
DENSITY: 0.0000126552145963607
NUM_NULLS: 0
NUM_BUCKETS: 4
LAST_ANALYZED: 2017/6/12 9:38:10
SAMPLE_SIZE: 5432
GLOBAL_STATS: YES
USER_STATS: NO
AVG_COL_LEN: 2
HISTOGRAM: FREQUENCY

--//因為bucket桶變成了4個,直方圖依舊是FREQUENCY.

[email protected]> select * from USER_TAB_HISTOGRAMS where table_name=‘T‘ and column_name=‘FLAG‘;
TABLE_NAME COLUMN_NAME ENDPOINT_NUMBER ENDPOINT_VALUE ENDPOINT_A
---------- ----------- --------------- ------------------------------------ ----------
T FLAG 2775 503652795277878000000000000000000000
T FLAG 5430 508845092136413000000000000000000000
T FLAG 5431 514037388994948000000000000000000000
T FLAG 5432 519229685853483000000000000000000000

[email protected]> alter session set statistics_level=all;
Session altered.

[email protected]> select * from t where flag=‘c‘;
ID PAD F
----- -------------------- -
40003 cccccccccccccccccccc c
40004 cccccccccccccccccccc c

[email protected]> @ &r/dpc ‘‘ ‘‘
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID g3pmd0h5vnw5r, child number 0
-------------------------------------
select * from t where flag=‘c‘

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 | | | 68 (100)| | 2 |00:00:00.01 | 204 |
|* 1 | TABLE ACCESS FULL| T | 1 | 7 | 196 | 68 (0)| 00:00:01 | 2 |00:00:00.01 | 204 |
--------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / [email protected]$1
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("FLAG"=‘c‘)
23 rows selected.

--//可以發現E-rows 與 A-rows 差距不大,而12c的情況就不同了.12c如果使用method_opt => ‘FOR ALL COLUMNS size repeat‘後,
--//bucket=2,直方圖變成了混合型直方圖.也就是如果升級12c,要註意分析方法method_opt => ‘FOR ALL COLUMNS size repeat‘帶來的問題.
--//再寫有點長,另外寫看12c的測試.

--//當然如果數據模型沒有發生變化,問題可以不考慮.

[20170612]FOR ALL COLUMNS SIZE repeat(11g).txt

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