Persistent iteration is performed in stages on nodes arranged according to a node identification (ID) that is increasing in order under a parent node in database tree structure. While performing the persistent iteration, storing a start node ID of the node where the persistent iteration commenced, a current node ID of the node that is undergoing the persistent iteration, a parent node ID of the node that is undergoing the persistent iteration, and a numerical value representing the current level and all levels greater than the current level of nodes that are undergoing the persistent iteration. Upon completion of the persistent iteration on the nodes that are the children of the node with the parent node ID, returning to the first node among the children of the node with the parent node ID, as ordered by increasing node ID, and performing the persistent iteration for children nodes of each node.