Redis 更新(set) key值过期时间被重置的问题：
问题描述：
当你在redis中插入一个key值，并且设置了对应过期时间.，当过期时间还没到的时候重新更新 key 值会导致过期时间被刷新。
针对这个问题 redis 的官方文档是这么解释的：
The timeout will only be cleared by commands that delete or overwrite the contents of the key, including DEL, SET, GETSET and all the *STORE commands. This means that all the operations that conceptually alter the value stored at the key without replacing it with a new one will leave the timeout untouched. For instance, incrementing the value of a key with INCR, pushing a new value into a list with LPUSH, or altering the field value of a hash with HSET are all operations that will leave the timeout untouched.
大致意思：
如果用DEL, SET, GETSET会将key对应存储的值替换成新的，命令也会清除掉超时时间；如果 list 结构中添加一个数据或者改变hset数据的一个字段是不会清除超时时间的；如果想要通过set去覆盖值那就必须重新设置expire。
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