Temporal connectives versus explicit

Some temporal query languages work directly on a timestamp repre sentation of the temporal database while others provide a more implicit access to the ow of time by means of temporal connectives We study the di erences 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 ver sions of rst order and xpoint logic respectively