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大表更改默认值使用alter table不重建表，直接修改.frm
在mysql中执行很大部分的修改动作，都需要重建一个表，然后把数据放进去，最后删除旧的表！有时候要是有索引的列上进行大批且频繁的表的时候会导致系统的性能严重下降，这里可以在修改SQL上做部分调整，减轻相关的构建结构带来的系统压力问题！ www.bitsCN.com
例如 在修改一个表的默认值为8的时候，常规做法为：
（1）：alter table modes modify column dept tinyint(3) not null default 8；
这里通过一些show status分析出，每次都要做大量的句柄的读取和插入操作，类似于从新构建了一张新表的样式，这样在一些大表，上千万的数据量会出现瓶颈问题！
这里我们需要灵活知道mysql的相关默认值实际是放在相关的表结构.frm文件中；我们可以不经过数据层，可以直接调整！上述的modify column会导致相关的表进行拷贝操作，不利于系统的正常稳定运行，可以使用下面方式；
（2）：alter table modes alter column dept set default 8;
这里只是更改了相关的frm文件，没有改动表，因此速度很快的即可完成；
总结：在此我们可以注意到相关的alter table后面跟不同形式的命令，可以对数据产生了不同程度的影响，这里还有一个change column操作也是不一样的！
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