MySQL的replace into分析
replace into和insert into是很相似的,唯一的差别就是使用replace into的时候会首先根据表的主键来判断是否存在与将要replace i
replace into和insert into是很相似的,唯一的差别就是使用replace into的时候会首先根据表的主键来判断是否存在与将要replace into的语句相同的记录,如果有的话,就先将旧记录删除,然后在插入新纪录。并且replace into执行后会返回一个数值,如果为1则表示执行的时候插入了一条数据,如果为2表示执行时删除了旧记录并且插入了新纪录。
虽然replace into在平常使用过程中不会出现什么严重的问题。如果我们要操作的表中有一个自增的主键,那么就要出问题了。首先,,因为新纪录与老记录的主键值不同,所以其他表中所有与本表老数据主键id建立的关联全部会被破坏。其次,就是,频繁的REPLACE INTO 会造成新纪录的主键的值迅速增大。总有一天。达到最大值后就会因为数据太大溢出了。就没法再插入新纪录了。数据表满了,不是因为空间不够了,而是因为主键的值没法再增加了。

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