Adding time dimension to relational model and extending relational algebra

A methodology for adding the time dimension to the relational model is proposed and relational algebra is extended for this purpose. We propose time-stamping attributes instead of adding time to tuples. Each attribute value is stored along with a time interval over which it is valid. Non-first normal form realations are used. A relation can have atomic, set-valued, triplet-valued, or set triplet-valued attributes. The last two types of attributes preserve the time (history). Furthermore, new algebraic operations are defined to extract information from historical relations. These operations convert one attribute type to another and do selection over the time dimension. Algebraic rules and identities for the new operations are also included.