We consider matrix index and navigation index approaches to in-memory navigation of persistent object stores. We demonstrate that both approaches can be re-formulated independently from the used coding technique. We expose the limitations of fixed length coding and the inefficiency of simple continued fractions as a variable length coding technique. The conclusion is that alternative variable length coding techniques, possibly based on known compression algorithms, should be explored. The architecture of persistent object stores should permit plugging in and configuring coding algorithms to optimise the operation of a particular system.