1. A machine-implemented method of collapsing a data tree hierarchy, the data tree hierarchy including leaf nodes and at least one root node, and each node including a system identification (SID), the method comprising: for each leaf node, at a leaf-node row: writing a leaf-node key value to a first column in a data table; and writing a corresponding leaf-node SID value to a second column in the data table; and at a root-node row: writing the leaf-node key value to the first column; and writing a corresponding root-node SID value to the second column, the entire data tree hierarchy collapsed into the first column and the second column, the second column having the leaf-node SID values representing a trail of traversal of the tree for each leaf node through intermediate nodes and to the at least one root node.