The '''nested set model''' is to number the nodes according to a [[tree traversal]], which visits each node twice, assigning numbers in the order of visiting, and at both visits. This leaves two numbers for each node, which are stored as two attributes. Querying becomes inexpensive: hierarchy membership can be tested by comparing these numbers. Updating requires renumbering and is therefore expensive. Refinements that use [[rational number]]s instead of integers can avoid renumbering, and so are faster to update, although much more complicated.<ref>{{cite arXiv |eprint=0806.3115 | first= Daniel|last = Hazel | title = Using rational numbers to key nested sets}}</ref>
