This field gives the number of times the root nodes have split
    because they overflowed after an insertion. A root node split
    causes the index to grow by one level-a parent node must be
    created to point to the two "halves" of the overflowed root node.
    Therefore, two nodes are created-the parent node and the node for
    the second half of the root node. Increasing the number of tree
    levels means Oracle Rdb must search more index nodes to access a
    data row; this can result in additional I/O operations.