【mysql】MyISAM表的存储格式_MySQL
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【mysql】MyISAM表的存储格式
MyISAM支持三种不同的存储格式：固定格式，动态格式，已压缩格式（只能使用myisampack工具来创建）。
静态表特征(固定格式)
静态格式是MyISAM表的默认存储格式。当表不包含变量长度列(VARCHAR, BLOB, 或TEXT)时,使用这个格式。每一行用固定字节数存储。静态格式是三种存储格式中最简单和最安全的，同时也是最快的ondisk格式。
基本特征：
(1)CHAR列对列宽度为空间填补。
(2)快速。
(3)容易缓存。
(4)由于每个记录位置固定，崩溃后容易重建。(5)无必要重新组织，通常比动态表需要更大的磁盘空间。
动态表特征(动态格式)
如果一个MyISAM表包含任何可变长度 列(VARCHAR, BLOB或TEXTDynamic),或者如果一个表被用ROW_FORMAT=DYNAMIC选项来创建,动态存储格式被使用。
基本特征：
(1)除了长度少于4的列之外，所有的字符串列存储为动态。
(2)每个记录前存储一个位图，该位图表明哪一列包含空字符串(对于字符串列)或者0(对于数字列)，但并不包括包含NULL值的列。而非空字符串列，则会被存为一个长度字节加字符串的内容。
(3)动态格式表在崩溃后要比静态格式表更难重建,因为一个记录可能被分为多个碎片且链接(碎片)可能被丢失。
(4)每个记录仅使用必需大小的空间。尽管如此,如果一个记录变大,它就按需要被分开成多片,造成记录碎片的后果。比如,你用扩展行长度的信息更新一行,该行就变得有碎片。在这种情况下,你可以时不时运行OPTIMIZE TABLE或myisamchk -r来改善性能。可使用myisamchk -ei来获取表的统计数据。
已压缩表特征
MySQL分发版本里默认包含myisampack工具，已压缩存储格式是由它创建的只读格式。
压缩表有下列特征：
(1)压缩表占据很小的磁盘空间，最小化了磁盘用量。
(2)每个记录被单独压缩，访问开支小。
(3)可以处理固定长度或动态长度记录。
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