Creation of branch nodes

Eventually, as you add rows to the table, the database server fills the root node with node pointers to all the existing leaf nodes. When the database server splits yet another leaf node, and the root node has no room for an additional node pointer, the following process occurs.

The database server splits the root node and divides its contents among two newly created branch nodes. As index items are added, more and more leaf nodes are split, causing the database server to add more branch nodes. Eventually, the root node fills with pointers to these branch nodes. When this situation occurs, the database server splits the root node again. The database server then creates yet another branch level between the root node and the lower branch level. This process results in a four-level tree, with one root node, two branch levels, and one leaf level. The B-tree structure can continue to grow in this way to a maximum of 20 levels.

Branch nodes can point either to other branch nodes below them (for large indexes of four levels or more) or to leaf nodes. In Figure 1, the branch node points to leaf nodes only. The first item in the left branch node contains the same key value as the largest item in the leftmost leaf node and a node pointer to it. The second item contains the largest item in the next leaf node and a node pointer to it. The third item in the branch node contains only a pointer to the next higher leaf node. Depending on the index growth, this third item can contain the actual key value in addition to the pointer at a later point during the life span of the index.
Figure 1. Typical Contents of a Branch Node
Begin figure description - The figure shows a branch node, labeled Branch node 2, and three leaf nodes labeled Leaf nodes 3, 4, and 5. The Branch node and each of the leaf nodes contain index items listed in two-column tables. Pointers extend from each of the branch node's index items to Leaf nodes 3, 4, and 5, respectively. End figure description

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