Temporal Connectives versus Explicit Timestamps in Temporal Query Languages

Some temporal query languages work directly on a timestamp representation of the temporal database, while others provide a more implicit access to the ow of time by means of temporal connectives. We study the diierences in expressive power between these two approaches. We rst consider rst-order logic (i.e., the relational calculus). We show that rst-order future temporal logic is strictly less powerful than the rela-tional calculus with explicit timestamps. We also consider extensions of the relational calculus with iteration constructs such as least xpoints or while-loops. We again compare augmentations of these languages with temporal left and right moves on the one hand, and with explicit times-tamps on the other hand. For example, we show that a version of xpoint logic with left and right moves lies between the explicit timestamp versions of rst-order and xpoint logic, respectively.