Disclosed herein are methods, systems, and apparatus, including computer programs encoded on computer storage media, for performing a map iteration by a network node of a blockchain network. One of the methods includes receiving a request to obtain a number of keys included in a map by the network node, the map storing a number of key-value pairs that include the number of keys and a number of values corresponding to the number of keys. The network node maintains data representing a forest that stores the number of keys that are stored in the map. The forest includes a number of trees, each tree includes up to a respective number of storage nodes, and each storage node stores a subset of the number of keys. The network node traverses the forest to retrieve the number of keys stored in the forest, and return the number of keys.