Oracle storage structure and relationships
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1、oracle以block 最小传输单位
2、nonstandard block size db_nk_cache_size n为 2的倍数,要与表空间块大小一样
3、block几个参数:
inittrans 块头上的事务槽,允许有多少事务同时访问块内的记录
maxtrans 最大事务数
pctfree 用于update预留的空间,小于pctfree 则从freelist删除
pctused 用于insert的预留的空间,小于pctused 重新加入freelist
4、auto segment-space management bitmap方式 只能在tablespace级设定,并只对local manage的有效,只有pctfree有效
5、manual data block management 主要如下参数: pctfree pctused freelist
6、HWM (HIGH-WATER MARK) 高水位,delete删除信息后并不移动指针,truncate恢复
7、pctfreee,pctused应用场景
high pctfree,low pctuesd :update and updates will increase the size of the rows frequently
low pctfree,high pctused :only insert or delete
8、表可能包含多个segment:主键索引 lob类型
9、sql跟踪 set autotrace on stat
10、几个数据字典
dba_extents
dba_segments
dba_tablespaces
dba_data_files
dba_free_space
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常用sql语句:
1、查找各个tablespace可用空间
select tablespace_name,count(*) as fragments,sum(bytes) as total,max(bytes) as largest from dba_free_space group by tablespace_name;
2、查找再申请extent会出错的segment
select s.segment_name,s.segment_type,s.tablespace_name,s.next_extent
from dba_segments s
where not exists(select 1
from dba_free_space f
where s.tablespace_name = f.tablespace_name
having max(f.bytes) >s.next_extent);
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