标签:相互 pre exe 解决 tis ase img oracle数据库 group
在Oracle数据库中,优化器欲产生最优的执行计划,需要依赖于相关对象的统计信息的搜集。数据库可以自动的搜集执行计划,也可以使用程序包DBMS_STATS进行手动搜集,本篇对常规统计信息的搜集不做说明,重点演示下关于扩展的统计信息(Extended Statistics)的的搜集。
在执行SQL时,where条件后的单个列的统计信息很容易去决定对应列的谓词的选择性,但是,如果where条件后包含同一张表的多个列作为谓词时,那么单个列的统计信息就不能显示出列之间的关系了,这时,优化器根据单列的统计信息得到的执行计划就可能不是最优的了。这个问题可以通过列组(column group)来解决,通过对列组进行统计信息的搜集,来反映一组列的相互关系,从而使优化器选择最优的执行计划,列组的统计信息称为扩展的统计信息。Oracle中,扩展的统计信息包含:
如图:展示了sh.customers表中两个列cust_state_province和contry_id列的统计信息,以及这两个列组成的列组的统计信息,列组的名称是由系统产生。
1)实验环境
SQL> select * from v$version;
BANNER CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production 0
PL/SQL Release 12.1.0.2.0 - Production 0
CORE 12.1.0.2.0 Production 0
TNS for Linux: Version 12.1.0.2.0 - Production 0
NLSRTL Version 12.1.0.2.0 - Production 0
2)构建测试数据
SQL> create table sh.customers_new as select * from sh.customers;
Table created.
SQL> exec dbms_stats.gather_table_stats(‘SH‘,‘CUSTOMERS_NEW‘,method_opt => ‘for all columns size 1‘);
PL/SQL procedure successfully completed.
3)查询列统计信息
SQL> select column_name,num_distinct,histogram
from dba_tab_col_statistics
where owner=‘SH‘ and table_name=‘CUSTOMERS_NEW‘ and column_name in(‘CUST_STATE_PROVINCE‘,‘COUNTRY_ID‘);
COLUMN_NAME NUM_DISTINCT HISTOGRAM
------------------------------ ------------ ---------------
CUST_STATE_PROVINCE 145 NONE
COUNTRY_ID 19 NONE
3)查询表的总数和满足cust_state_province=‘CA‘的数据量
SQL> select count(1) from sh.customers_new;
COUNT(1)
----------
55500
SQL> select count(1) from sh.customers_new where cust_state_province=‘CA‘;
COUNT(1)
----------
3341
4)查看单列对应的执行计划
SQL> explain plan for
2 select * from sh.customers_new where cust_state_province=‘CA‘;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3410015392
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 383 | 72387 | 423 (1)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| CUSTOMERS_NEW | 383 | 72387 | 423 (1)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------
1 - filter("CUST_STATE_PROVINCE"=‘CA‘)
13 rows selected.
看下面的计算,和优化器预估的行数相匹配:
SQL> select 55500/145 from dual;
55500/145
----------
382.758621
5)查看组合条件对应的执行计划
SQL> explain plan for
2 select * from sh.customers_new where cust_state_province=‘CA‘ and country_id=‘52790‘;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3410015392
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 20 | 3780 | 423 (1)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| CUSTOMERS_NEW | 20 | 3780 | 423 (1)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------
1 - filter("CUST_STATE_PROVINCE"=‘CA‘ AND "COUNTRY_ID"=52790)
13 rows selected.
结果显示,优化器预估的行数为20行,和下面的计算相同,而实际的行数却为3341行,为何会这样呢?
SQL> select 55500/145/19 from dual;
55500/145/19
------------
20.1451906
首先,优化器不知道这两列的关系,不知道CA在国家52790,从而大大低估了返回的数据行;
通过上面的实验,可以得知,如果列之间有一定的相互关系,且where条件出现组合条件时,优化器不能准确的评估返回的行数,那么对于这种情况,可以对组合列进行统计信息的搜集。
1)搜集组合列统计信息
begin
dbms_stats.gather_table_stats(‘SH‘,
‘CUSTOMERS_NEW‘,
method_opt => ‘for all columns size 1,for columns (cust_state_province,country_id) size skewonly‘);
end;
/
PL/SQL procedure successfully completed.
2)查看组合条件对应的执行计划
explain plan for
select * from sh.customers_new where cust_state_province=‘CA‘ and country_id=‘52790‘;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3410015392
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3341 | 655K| 423 (1)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| CUSTOMERS_NEW | 3341 | 655K| 423 (1)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------
1 - filter("CUST_STATE_PROVINCE"=‘CA‘ AND "COUNTRY_ID"=52790)
13 rows selected.
可以看出预估的行数为3341,而实际的行数为3341,和真实结果一样。
SQL> select count(1) from sh.customers_new where cust_state_province=‘CA‘ and country_id=‘52790‘;
COUNT(1)
----------
3341
3)删除扩展统计信息
begin
dbms_stats.drop_extended_stats(‘sh‘,
‘CUSTOMERS_NEW‘,
‘(cust_state_province, country_id)‘);
end;
/
PL/SQL procedure successfully completed.
以上,就是对扩展统计信息中列组统计信息的演示。
【12c】带你了解Oracle 12c数据库扩展的统计信息(Extended Statistics)的搜集
标签:相互 pre exe 解决 tis ase img oracle数据库 group
原文地址:https://www.cnblogs.com/alen-liu-sz/p/12975555.html