mysql手记_MySQL
myisam innoDB是mysql常用的存储引擎
MyISAM不支持事务、也不支持外键,但其访问速度快,对事务完整性没有要求。
InnoDB存储引擎提供了具有提交、回滚和崩溃恢复能力的事务安全。但是比起MyISAM存储引擎,InnoDB写的处理效率差一些并且会占用更多的磁盘空间以保留数据和索引。
innodb的索引有两种,叫第一索引,以及第二索引。有的也叫聚集索引与辅助索引。其中聚集索引存放了表中的记录,查询的时候不需要回表扫描,同时索引项较大;辅助索引存放的位置信息,需要回表扫描,相对来说,I/0 次数会增加。
查询的时候最好能够从索引中取得数据,减少回表,相对来说离散的 I/0,
MYISAM 没有聚集索引。存放的记录的物理位置
OLTP (联机事务处理)故名思议主要强调事务,如(银行存款的修改,用户订单等)面向应用
OLAP (联机分析处理) 主要作为数据仓库,面向决策,分析等。
联接算法:
nested-loops join 主要思想是:从外表中拿出一个数据与内表的每一条数据比较,O(M*N) 。当有索引时:内表只需要比较索引的高度,近似于O(M*H)
Block nested--loops join 主要思想 是:改进 nested-loops join 外部表每次去一定的数据到缓冲区,比如10条,然后这10条记录在跟内部表的数据比较,减少内部表的扫描次数。
Hash join 只能== 以及!=,不能部分比较(为何?,hash是对整个字符串hash) 主要思想是:将外部表的数据放到join buffer,然后hash,这一阶段
为build;probe阶段,从内表中取出数据hash,比较。
基本的测试:
create TABLE myorder
( id int not null auto_increment,
userid int not null ,
orderdate date,
comein int DEFAULT 0,
comeout int DEFAULT 0,
PRIMARY key (id)
);
SELECT userid ,orderdate ,comein -comeout as rest
from myorder
GROUP BY userid ,orderdate;
create index myorderindex on myorder(id,userid);
explain SELECT userid ,orderdate ,comein -comeout as rest
from myorder
GROUP BY userid ,orderdate;

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