## B-Trees Concepts B-树介绍（都快忘了:))

Although most of the search trees are binary trees, there is a popular serach tree taht is not binary. this tree is known as a B-tree.

A B-tree of order M is a tree with the following structural properties:

(1) Theroot is either a leaf or has between 2 and M children.
(2) All nodeleaf nodes(except the root) have between [M/2] and M children.
(3) All leaves are at the same depth.
(4) Each node should contain such data fields
(P0,K0,P1,K1,....Pi,Ki..Pm,Km)
[K0,K1,...Km] is the increased key.
[P0,P1,...Pm] is the pointer which is point to its related child nodes.

（1）根节点要么是叶子节点 或者有2到m 个孩子节点。
(2) 所有非叶子节点有 m/2到m个孩子节点。
(3)所有叶子节点都具有相同高度。
(4) 每个节点应含有以下数据域：
(P0,K0,P1,K1,....Pi,Ki..Pm,Km)
其中， [K0,K1,...Km] 是递增的关键字序列
[P0,P1,...Pm] 是指向其孩子节点的指针域，初始为空。

All data are stored at the leaves. Contained in each interios node are pointers P1,P2,...Pm to the children, and values k1,k2,...k(m-1),representing the smallest key found in the subtrees P2,P3,...,Pm,respectively. Of course, some of the these pointers might be NULL, and the corresponding Ki would then be undefined. For every node, all the keys in subtree P1 are smaller than the keys in subtree P2, and so on. The leaves contail all the actula data, whichare either the keys themselves or pointers to recores containing the keys. We will assume the former tokeep our examples simpe. There are various definitions of B-trees that change this structure in mostly minor ways, but this definition is one ot the popular forms. We will also insist that the number of keys in a leaf is also between
[M/2] and M.

The tree as following diagram showed is an example of a B-tree of order 4 A B-tree of order 4 is more popularly known as a 2-3-4 tree, and a B-tree of order 3 is know as  a 2-3 tree. we will describe the operation of B-trees by using the special case of 2-3 trees. our starting point is the 2-3 tree follows. We have drawn interior nodes(nonleaves) , which contain the two pieces of data for each node. A dash line as a second piece of information in an interior node indicates that the node has only tow children. Leaves are drawn in boxes, which contain the keys. The keys in the leaves are ordered. To perform a Find. we start at the root and branch in one of(at most) three directions, depending on the relation of the key we are looking for to the two (possibly one) values stored at the node

To perform an Insert on a previously unseen key, X, we follow the path as though we were performing a Find. when we get to a leaf node, we have found the correct place to put X. Thus, to insert a node with key 18, we can just add it to a leaf without causing any violations of the 2-3 tree properties. The result is shown in the following figure.

Unfortunately, since a leaf can hold only two or three keys,this might not always be possible, If we now try to insert 1 into the tree, we find that the node where it belongs is already full. Placing our new key into this node would give it a fourth element, which is not allowed. This is can be solved by making two nodes of two keys each and adjusting the information in the parent.

Unfortanately, this idea does not always work, as can be seen by an attempt to insert 19 into the current tree. If we make two nodes of two keys each, we obtain the following tree. This tree has an internal node with four children, but we only allow three per node. The solution is simple,
We merely split this node into two nodes with two children. of course, this node might be one of three children itself, and thus splitting it would create a probrem for its parent(which would now have four children), but we can keep on splitting nodes on the way up to the root until we either get to the root or find a node with only two children. posted on 2008-04-28 17:41  飞天舞者  阅读(650)  评论(0编辑  收藏