B-Tree Structure
Index node is a sequence of [pointer, key] pairs
K1 < K2 < … < Kn-2 < Kn-1
P1 points to a node containing keys < K1
Pi points to a node containing keys in range [Ki-1, Ki)
Pn points to a node containing keys > Kn-1
So, K ´1 < K ´2 < … < K ´n-2 < K ´n-1