mysql存储引擎(一)
mysql各种存储引擎(一) mysql各种存储引擎一 MyISAM MyISAM文件格式 MyISAM文件修复 MyISAM表的存储格式 Innodb存储引擎 支持事物 自动增长列 外键约束 存储结构 MyISAM MyISAM文件格式 frm文件 存储表的结构定义数据 MYD文件 具体的数据文件 MYI文件 表索
mysql各种存储引擎(一)
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- mysql各种存储引擎一
- MyISAM
- MyISAM文件格式
- MyISAM文件修复
- MyISAM表的存储格式
- Innodb存储引擎
- 支持事物
- 自动增长列
- 外键约束
- 存储结构
- MyISAM
- mysql各种存储引擎一
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MyISAM
MyISAM文件格式
- frm文件 存储表的结构定义数据
- MYD文件 具体的数据文件
- MYI文件 表索引文件
myISAM存储引擎只缓存索引,对数据文件采用操作系统缓存,如果索引数据超过系统所分配的缓存空间时也会采用操作系统来缓存索引.
MyISAM文件修复
- 通过使用check table语句检测MyISAM表的状态,然后用REPAIR TABLE语句修复损坏的表
- 使用myisamchk工具修复,如果有用户同时在操作这张表,那么即使表是没有问题的,也很有可能提示表已损坏,故进行检查时,应当先停掉mysql服务。如果不想停止服务,至少应该mysqladmin flush-tables操作。
<code class=" hljs haml"> [? JavaStudy git:(master) myisamchk -e books.MYI Checking MyISAM file: books.MYI Data records: 0 Deleted blocks: 0 -<span class="ruby"> check file-size </span> -<span class="ruby"> check record delete-chain </span> -<span class="ruby"> check key delete-chain </span> -<span class="ruby"> check index reference </span> -<span class="ruby"> check data record references <span class="hljs-symbol">index:</span> <span class="hljs-number">1</span> </span> -<span class="ruby"> check records <span class="hljs-keyword">and</span> index references </span></code>
MyISAM表的存储格式
- 静态(固定长度)表 默认格式,固定非变长字段,存储速度快,容易缓存,易恢复,缺点所占空间较多,且利用率不高
- 动态可变长度表,可变长存储,恢复困难,行变化较大时会被分成碎片,需要使用myisamchk -r 来修复
- 压缩表 存储空间最小,每行进行压缩,使用数据库提供的myisanpack工具压缩。
Innodb存储引擎
支持事物
4个事物级别:
- read uncommitted 最低级别,以下情况均无法保证
- read committed 可避免脏读情况发生
- repeatable read 可避免脏读、不可重复读情况的发生 数据库的默认级别
- serializable 可避免脏读、不可重复读、虚读情况的发生
自动增长列
auto_increment属性,表在添加数据的时候,可以插入空值,该列可以自动增加数据
外键约束
它降低了数据库的查询效率,数据库表之间的耦合度更加紧密,要求父表必须有相对应的索引,字表在创建时回添加相应的索引。
存储结构
- .frm文件 数据的结构定义文件
- .ibd文件 数据与索引文件
(1)使用共享表空间
所有的表盒索引数据存放在同一个表空间中,数据和索引在innodb_data_home_dir和innodb_data_file_path定义的表空间中,可以使用一个或多个数据文件
(2)使用多表空间
表结构存放在.frm文件中,但是每个表的数据和索引文件存放在一个单独的.ibd文件中.要使用多表空间存储时,需要设置innodb_file_per_table参数,这样每个数据库的表都会生成一个数据空间
查看如下:
<code class=" hljs lua"> mysql> show variables like <span class="hljs-string">'%per_table%'</span>; +<span class="hljs-comment">-----------------------+-------+</span> | Variable_name | Value | +<span class="hljs-comment">-----------------------+-------+</span> | innodb_file_per_table | ON | +<span class="hljs-comment">-----------------------+-------+</span> <span class="hljs-number">1</span> row <span class="hljs-keyword">in</span> set (<span class="hljs-number">0.00</span> sec) </code>
修改前需要关闭数据库,然后在配置文件my.cnf中设置或者添加该参数innodb_file_per_table = 1,重启数据库服务。

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