MySql 中的 Next-key lock 怎么工作的？

先帖一段官方的解释： A next-key lock on an index record also affects the “ gap ” before that index record. That is, a next-key lock is an index-record lock plus a gap lock on the gap preceding the index record. If one session has a shared or exclusive lock on record R in an index, another session cannot insert a new index record in the gap immediately before R in the index order.

假设我有一张表有一些数据 1,3,5,8,11 且具有非唯一索引，并且在 repeatable read 级别下执行：

select * from table where col = 8 for update;

此时会锁住 8 这条记录，另外的 gap 会锁住哪个范围？实际测试其实会锁住(5,8) 和 (8,11)这两个 gap 。但是根据官方手册里，不是before, preceding the index record 吗?（ 8 ， 11 ）这个 gap 哪来的啊？已经纠结很久这个问题了，希望哪位大神帮忙解答下，不胜感激啊 :D