B-trees are special m-ary balanced trees utilized in databases since their structure allows records to be added, deleted & retrieved with guaranteed worst case performance.

A B-Tree is a special multiway tree. In a B-Tree each of nodes may contain a large number of keys. The number of subtrees of each of node may also be large. A B-Tree is designed to branch out in this great number of directions and to contain many keys in each node so that height of the tree is comparatively small.

It means that only a small number of nodes have to be read from disk to retrieve an item.

A B-Tree of order m is multiway search tree of order m such as

• All leaves are onto the bottom level
• All internal nodes (except root node) contain at least m/2 (non empty) children
• The root node can have as few as 2 children if this is an internal node and can have no children if the root node is leaf node
• Each leaf node has to contain atleast (m/2) - 1 keys. The given is the structure for a B-tree :

struct btree

{          int count;         // number of keys stored into the current node

            item_type key[3];        // array to hold 3 keys

            long branch [4];           // array of fake pointers (records numbers)

};                    

Figure depicts an order 5 B-tree.

Figure: A B-tree of order 5