oracle统计信息-11新特性多列统计信息

当where子句后指定了一个表的多个列条件时，优化器通常会将多个列的选择性（selectivity）相乘得到where语句的选择性，导致优化器做出错误判断！

Oracle 11g引入了多列统计信息概念，如果上面情况列关联性很好，我们可以做多列统计信息收集，让优化器做出正确判断。

例如：

SQL> select count(*) from sh.customers where CUST_STATE_PROVINCE = 'CA';

COUNT(*)

----------

3341

SQL> select count(*) from sh.customers where CUST_STATE_PROVINCE = 'CA' and country_id=52790;

COUNT(*)

----------

3341

SQL> select count(*) from sh.customers where CUST_STATE_PROVINCE = 'CA' and country_id=52775;

COUNT(*)

----------

可以看出表customers的列CUST_STATE_PROVINCE、country_id是有关系的，只有多列统计信息才能让优化器知道他们的关系，从而有一个更精确的选择性（selectiveity）。

1、创建Column Group：

DECLARE

cg_name varchar2(30);

BEGIN

cg_name := dbms_stats.create_extended_stats(null,'customers',

'(cust_state_province,country_id)');

END;

2、查看column group name:

select sys.dbms_stats.show_extended_stats_name('sh','customers',

'(cust_state_province,country_id)') col_group_name

from dual;

COL_GROUP_NAME

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

SYS_STU#S#WF25Z#QAHIHE#MOFFMM_

3、删除Column Group

exec dbms_stats.drop_extended_stats('sh','customers','(cust_state_province,country_id)');

4、监控Column Groups

--查询多列统计信息

Select extension_name, extension

from user_stat_extensions

where table_name='CUSTOMERS';

EXTENSION_NAME EXTENSION

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

SYS_STU#S#WF25Z#QAHIHE#MOFFMM_ ("CUST_STATE_PROVINCE","COUNTRY_ID")

--查看distinct数和柱状图使用情况

select e.extension col_group, t.num_distinct, t.histogram

from user_stat_extensions e, user_tab_col_statistics t

where e.extension_name = t.column_name

and e.table_name = t.table_name

and t.table_name = 'CUSTOMERS';

COL_GROUP NUM_DISTINCT HISTOGRAM

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

("CUST_STATE_PROVINCE","COUNTRY_ID") 145 FREQUENCY

5、收集多列统计信息

1）收集已存在的列组统计信息

EXEC DBMS_STATS.GATHER_TABLE_STATS('SH','CUSTOMERS',METHOD_OPT =>'FOR ALL COLUMNS SIZE AUTO');

2）收集新指定的列组统计信息

EXEC DBMS_STATS.GATHER_TABLE_STATS('SH','CUSTOMERS',METHOD_OPT =>'FOR ALL COLUMNS SIZE SKEWONLY FOR COLUMNS (CUST_STATE_PROVINCE,COUNTRY_ID) SIZE SKEWONLY');

实验：

1）当不使用多列统计信息时，真实结果是3341，执行计划是1107.

SQL> exec dbms_stats.drop_extended_stats('sh','customers','(cust_state_province,country_id)');

SQL> select count(*) from sh.customers where CUST_STATE_PROVINCE = 'CA' and country_id=52790;

COUNT(*)

----------

3341

Execution Plan

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

Plan hash value: 296924608

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

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

| 0 | SELECT STATEMENT | | 1 | 16 | 406 (1)| 00:00:05 |

| 1 | SORT AGGREGATE | | 1 | 16 | | |

|* 2 | TABLE ACCESS FULL| CUSTOMERS | 1107 | 17712 | 406 (1)| 00:00:05 |

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

Predicate Information (identified by operation id):

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

2 - filter("CUST_STATE_PROVINCE"='CA' AND "COUNTRY_ID"=52790)

Statistics

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

120 recursive calls

0 db block gets

1588 consistent gets

1454 physical reads

0 redo size

423 bytes sent via SQL*Net to client

420 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

7 sorts (memory)

0 sorts (disk)

1 rows processed

2）当使用多列统计信息时，真实结果是3341，执行计划是3294.

SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS('SH','CUSTOMERS',METHOD_OPT =>'FOR ALL COLUMNS SIZE SKEWONLY FOR COLUMNS (CUST_STATE_PROVINCE,COUNTRY_ID) SIZE SKEWONLY');

SQL> select count(*) from sh.customers where CUST_STATE_PROVINCE = 'CA' and country_id=52790;

COUNT(*)

----------

3341

Execution Plan

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

Plan hash value: 296924608

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

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

| 0 | SELECT STATEMENT | | 1 | 16 | 406 (1)| 00:00:05 |

| 1 | SORT AGGREGATE | | 1 | 16 | | |

|* 2 | TABLE ACCESS FULL| CUSTOMERS | 3294 | 52704 | 406 (1)| 00:00:05 |

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

Predicate Information (identified by operation id):

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

2 - filter("CUST_STATE_PROVINCE"='CA' AND "COUNTRY_ID"=52790)

Statistics

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

8 recursive calls

0 db block gets

1458 consistent gets

1454 physical reads

0 redo size

423 bytes sent via SQL*Net to client

420 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

3）即以上情况，使用多列统计信息能让优化器得到更准确的判断！

来源：CSDN

作者：执笔画情ora

链接：https://blog.csdn.net/oradbm/article/details/103882116

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