3. Next-Key Locks
是Gap锁 与 Record Locks的结合，区间为前开后闭。如一个index 有10, 11, 13, and 20几条记录，则可能存在的Next-Key Locks是：
(negative infinity, 10]
(10, 11]
(11, 13]
(13, 20]
(20, positive infinity)
Next-Key Locks用于RR事务隔离级别，解决幻读问题。
mysql locking
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