B树

介绍B树之前，先介绍树的几个概念。

Degree

　　The number of subtrees of a node.

Height of node

　　The height of a node is the number of edges on the longest path between that node and a leaf.

Height of tree

　　The height of a tree is the height of its root node.

Depth

　　The depth of a node is the number of edges from the tree's root node to the node.

定义

A B-Tree is defined by the term minimum degree ‘t’. The value of t depends upon disk block size.

Every node except root must contain at least t-1 keys. Root may contain minimum 1 key.
All nodes (including root) may contain at most 2t – 1 keys.
Number of children of a node is equal to the number of keys in it plus 1.
All keys of a node are sorted in increasing order.
All leaves are at same level.

另一种定义方式