CME: a temporal relational model for efficient coalescing

Coalescing is a data restructuring operation applicable to temporal databases. It merges timestamps of adjacent or overlapping tuples that have identical attribute values. The likelihood that a temporal query employs coalescing is very high. However, coalescing is an expensive and time consuming operation. In this paper, we present a novel temporal relational model through which coalescing becomes quite simple. The basic idea is to augment each time-varying attribute in a temporal relation with two additional attributes that trace changes in values of the corresponding time-varying attribute. One attribute traces changes in values with respect to each individual instance (i.e. tuples having the same key value), while the other attribute traces changes in values globally for all instances (i.e. all tuples in the temporal relation). Using these tracing attributes, coalescing could be easily implemented through a quite simple join-free group-by query. The coalescing query is fully processed and optimized by the underlying database management system.