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

Posted 2020-09-22 lfree

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

--//昨天看了https://jonathanlewis.wordpress.com/2017/06/01/histogram-upgrade-2/,提到了直方图的问题,
--//特别是FOR ALL COLUMNS SIZE repeat引起的问题,在一些特殊情况要注意.

1.环境:
[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

--//建立了直方图.

[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

--//如果应用模式发生变化,加入了2种值.

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

--//因为bucket桶变成了4个,直方图依旧是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.

--//可以发现E-rows 与 A-rows 差距不大,而12c的情况就不同了.12c如果使用method_opt => ‘FOR ALL COLUMNS size repeat‘后,
--//bucket=2,直方图变成了混合型直方图.也就是如果升级12c,要注意分析方法method_opt => ‘FOR ALL COLUMNS size repeat‘带来的问题.
--//再写有点长,另外写看12c的测试.

--//当然如果数据模型没有发生变化,问题可以不考虑.