Adding time dimension to relational model and extending relational algebra (Q1094156)

A new method of incorporating time information into database relations is proposed. While earlier approaches attached time-instant or -interval annotations to tuples, this method (optionally) attaches begin and end times to individual attribute values. The model also allows attribute values that are sets of basic values or time-annotated basic values. The advantage over other approaches is that time-series data on an entity can be collected into a single tuple, rather than spread over multiple tuples, where key and interval ordering considerations complicate extracting time-series data on a single entity. The presence of set- valued attributes means relations in the model are not in first normal form. However, the model here is not as general as other non-first normal models, since nested sets are prohibited. Possibly a simpler system and exposition would have resulted if set constructs were independent of time-attachment, and nesting of sets were allowed. The bulk of the paper is definitions of operations in a relational algebra over time-annotated relations. There are analogs of standard algebraic operators (select, project, join, Cartesian product, union, intersection, difference) that deal with time intervals and set-valued attributes. Nest and unnest operations convert between a set-valued attribute and a collection of tuples with simple attribute values. Other operations convert time annotations to independent attributes and vice- versa. The algebra also contains aggregation operations, and operations to split and combine time intervals. The paper concludes with identities on expressions over the algebra.