微课sql优化(2)-为什么需要收集统计信息
1、为什么需要收集统计信息
Cost-based optimizer (CBO)，Based on object statistics，优化器统计信息描述数据库中的对象的详细信息，
查询优化器使用这些统计信息为每个SQL语句选择最佳执行计划。
数据库将优化器统计信息存储在数据字典中。您可以使用数据字典视图访问这些统计信息。
由于数据库中的对象可能会不断更改，因此必须定期更新统计信息，以便准确描述这些对象。Oracle数据库自动维护优化器统计信息。
11g：
Scheduled Maintenance Window Times
，
In 11g daily maintenance windows are provided. by default these are defined as :
Weeknights: Starts at 10 p.m. and ends at 2 a.m.
Weekends: Starts at 6 a.m. is 20 hours long.
10g：
With Oracle Database 10g the default maintenance window is configured to cover the following periods:
10 pm to 6 am every weekday
All weekend (Friday 10 pm to Monday 6 am)
可以使用该DBMS_STATS软件包手动维护优化器统计信息。
2、哪些表需要收集统计信息
自动统计任务能满足大部分数据库的业务需求，但存在特殊情况需要手工收集统计信息，
1、业务表数据被delete、truncate、impdp/imp大批量数据变理时。
2、数据变更量超过10%时。
查询哪些表需要收集统计信息
col table_name for a30
col object_type for a30
select owner, table_name, object_type, stale_stats, to_char(LAST_ANALYZED,'yyyy-mm-dd hh34:mi') last_analyzed
from dba_tab_statistics
where owner = 'HT'
and (stale_stats = 'YES' or last_analyzed is null);
3、练习题1、测试统计信息阈值： 变更量超过10%
create table ht.test as select * from dba_objects;
exec DBMS_STATS.GATHER_TABLE_STATS(ownname => 'HT',tabname => 'TEST',method_opt => 'for all columns size repeat',no_invalidate => FALSE,degree => 4,cascade => TRUE);
SQL> DELETE FROM ht.TEST WHERE ROWNUM<=(select count(1) from ht.TEST)*0.1;
8632 rows deleted.
SQL> commit;
Commit complete.
exec dbms_stats.flush_database_monitoring_info; --刷新变更信息
set line 200
col table_owner for a10
col table_name for a30
col pname for a20
col spname for a20
select table_owner,table_name,partition_name pname,subpartition_name spname,inserts,updates,deletes from dba_tab_modifications
where table_owner='HT';
TABLE_OWNE TABLE_NAME PNAME
SPNAME INSERTS
UPDATES DELETES
------
---
---
---
---- ---------------- ----------- --------------
--
--
----------
--
----------
--
--
--
----------
HT TEST
0 0 8632
col table_name for a30
col object_type for a30
COL last_analyzed for a20
select owner, table_name, object_type, stale_stats, to_char(LAST_ANALYZED,'yyyy-mm-dd hh34:mi') last_analyzed
from dba_tab_statistics
where owner = 'HT' AND TABLE_NAME='TEST';
OWNER TABLE_NAME OBJECT_TYPE STALE_STA LAST_ANALYZED
---------- ---------------------- ------------------------------ --------- --------------------
HT TEST TABLE NO 2017-08-18 21:26
DELETE FROM ht.TEST WHERE ROWNUM=1;
exec dbms_stats.flush_database_monitoring_info; --刷新变更信息
col table_name for a30
col object_type for a30
COL last_analyzed for a20
select owner, table_name, object_type, stale_stats, to_char(LAST_ANALYZED,'yyyy-mm-dd hh34:mi') last_analyzed
from dba_tab_statistics
where owner = 'HT' AND TABLE_NAME='TEST';
4、小结
哪些情况需要从新收集统计信息,
1、业务表数据被delete、truncate、impdp/imp大批量数据变更时。
2、数据变更量超过10%时,统计信息状态标记为过期
。
