B+ Tree
Example
segmentfault 由 B-/B+树看 MySQL索引结构
geeksforgeeks Introduction of B+ Tree
In order, to implement dynamic multilevel indexing, B-tree and B+ tree are generally employed. The drawback of B-tree used for indexing, however is that it stores the data pointer (a pointer to the disk file block containing the key value), corresponding to a particular key value, along with that key value in the node of a B-tree. This technique, greatly reduces the number of entries that can be packed into a node of a B-tree, thereby contributing to the increase in the number of levels in the B-tree, hence increasing the search time of a record.
NOTE:
一、search tree的搜索时间是由树高决定的
其实，它在此处的介绍都是非常不细致的，在下面的文章中进行了非常细致的介绍:
codinglabs MySQL索引背后的数据结构及算法原理