Querying now-relative data

Abstract

Now-relative temporal data play an important role in most temporal applications, and their management has been proved to impact in a crucial way the efficiency of temporal databases. Though several temporal relational approaches have been developed to deal with now-relative data, none of them has provided a whole temporal algebra to query them. In this paper we overcome such a limitation, by proposing a general algebra which is polymorphically adapted to cope with the MAX "reference" approach, and with our POINT approach (and that is easily adaptable to cope with other relational approaches to now-relative data in the literature, such as the NULL and the MIN ones). Besides being general enough to provide a query language for several approaches in the literature, our algebra has been designed in such a way to satisfy several theoretical and practical desiderata: closure with respect to representation languages, correctness with respect to the "consensus" BCDM semantics, reducibility to the standard non-temporal algebra (which involves interoperability with non-temporal relational databases), implementability and efficiency. Indeed, the experimental evaluation we have drawn on our implementation has shown that only a slight overhead is added by our treatment of now-relative data (with respect to an approach in which such data are not present).

A Comprehensive Approach to ‘Now’ in Temporal Relational Databases: Semantics and Representation

“…More recently, the POINT approach has been proposed [18], which outperforms the MAX, MIN and NULL approaches. Even more recently, a relational algebra has been defined to fully support querying NOW-related data (i.e., without resorting to the instantiation to the current time) in all MAX, MIN, NULL and POINT approaches [10]. All such approaches are (implicitly) based on Clifford et al's semantics, assuming latency equal to zero.…”

“…At RT=11, the semantics of Example 4 is <John, ICU | { (10,10), (10,11), (11,10), (11,11)}>, and at RT=12 it becomes <John, ICU | { (10,10), (10,11), (10,12), (11,10), (11,11), (11,12), (12,10), (12,11), (12,12)}>.…”

“…For instance, in case NOW-1 is used instead of NOW, the semantics of Example 4 at RT=12 is: <John, ICU | { (10,10), (10,11), (11,10), (11,11), (12,10), (12,11) …”

“…We are not aware of any other algebra explicitly coping with now-related facts (except the POINT approach [10][18], which, however, does not explicitly provide any semantics for NOW). Thus, we have chosen to compare the performance of our approach with an "ideal" (but not realistic) approach in which the exact ending time of nowrelative data is known a priori.…”

“…Such representational approaches adopt indexing techniques to enhance efficiency, and are experimentally evaluated and compared [8] [9]. An algebra for such approaches have been recently proposed by Anselma et al [10]. However, their data and query semantics has not been explicitly explored yet.…”