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有同学问到MySQL的uuid这个函数。简要介绍一下。用法
简单看到，这个值，每次执行都是不同的。 生成规则第1 2 3 段是与时间有关的。 
time_low、time_mid、time_high_and_version转成16进制后分别对应第1 2 3段。这个时间是从1582-10-15 00:00:00.00到当前时间的100ns值。（实际上系统只能取到精确us，再乘以10）。所以你短时间连续执行的话，比较可能只有第一个值在改，实际上1 2 3都可能会改变。 第4段是你启动这个MySQL后第一次执行select uuid()时的随机数，每次重启会改变。 第5段是mac值转过来的，同一个机器多实例的一般相同。如果mac值获取不到，则是一个随机值。 所以这个值可以认为是每次执行都不相同。并且不同实例之间也只有极微小概率重复。 Uuid_short
与uuid返回固定长度字符串不同， uuid_short的返回值是一个unsigned long long类型。MySQL启动后第一次执行的值是通过server_id
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