24 research outputs found
ON THE EXPRESSIVE POWER OF INFINITE TEMPORAL DATABASES
We discuss different techniques for representing infinite temporal data. There are
two basic approaches: A procedural description, as used in production systems, and
represented, in this paper, by a version of Datalog. The second approach is a more
declarative method, using some form of temporal logic programming. We examine several
versions of each approach, and compare their expressive power, i.e., what temporal
data each formalism can capture.Information Systems Working Papers Serie
Irregular Indeterminate Repeated Facts in Temporal Relational Databases
Time is pervasive of reality, and many relational database approaches have been developed to cope with it. In
practical applications, facts can repeat several times, and only the overall period of time containing all the repetitions may be
known (consider, e.g., On January, John attended five meetings of the Bioinformatics project). While some temporal relational
databases have faced facts repeated at (known) periodic time, or single facts occurred at temporally indeterminate time, the
conjunction of non-periodic repetitions and temporal indeterminacy has not been faced yet. Coping with this problem requires
an in-depth extension of current techniques. In this paper, we have introduced a new data model, and new definitions of
relational algebraic operators coping with the above issues. We have studied the properties of the new model and algebra (with
emphasis on the reducibility property), and how it can be integrated with other models in the literature
On decidability and tractability of querying in temporal EL
We study access to temporal data with TEL, a temporal extension of the tractable description logic EL. Our aim is to establish a clear computational complexity landscape for the atomic query answering problem, in terms of both data and combined complexity. Atomic queries in full TEL turn out to be undecidable even in data complexity. Motivated by the negative result, we identify well-behaved yet expressive fragments of TEL. Our main contributions are a semantic and sufficient syntactic conditions for decidability and three orthogonal tractable fragments, which are based on restricted use of rigid roles, temporal operators, and novel acyclicity conditions on the ontologies
Nearly Periodic Facts in Temporal Relational Databases
Despite the huge amount of work devoted to the treatment of time within the relational context, few relevant
temporal phenomena still remain to be addressed. One of them is the treatment of \u201cnearly periodic events\u201d, i.e., eventsacts
that occur in intervals of time which repeat periodically (e.g., a meeting occurring twice each Monday, possibly not at regular
times). Nearly periodic events are quite frequent in everyday life, and thus in many applicative contexts. Their treatment within
the relational model is quite challenging, since it involves the integrated treatment of three aspects: (i) the number of repetitions,
(ii) their periodicity, and (iii) temporal indeterminacy. Coping with this problem requires an in-depth extension of current temporal
relational database techniques. In this paper, we introduce a new data model, and new definitions of relational algebraic
operators coping with the above issues. We ascertain the properties of the new model and algebra, with emphasis on the
expressiveness of our representation model, on the reducibility property, and on the correctness of the algebraic operators
A TEMPORAL RELATIONAL ALGEBRA AS A BASIS FOR TEMPORAL RELATIONAL COMPLETENESS
We define a temporal algebra that is applicable to any
temporal relational data model supporting discrete linear
bounded time. This algebra has the five basic
relational algebra operators extended to the temporal
domain and an operator of linear recursion. We
show that this algebra has the expressive power of a
safe temporal calculus based on the predicate temporal
logic with the until and since temporal operators.
In [CrC189], a historical calculus was proposed as a
basis for historical relational completeness. We propose
the temporal algebra defined in this paper and
the equivalent temporal calculus as an alternative basis
for temporal relational completeness.Information Systems Working Papers Serie
MODELING DYNAMICS OF DATABASES WITH RELATIONAL DISCRETE EVENT SYSTEMS AND MODELS
Behavior of relational databases is studied within the framework of Relational Discrete Event
Systems (RDESes) and Models (RDEMs). Three behavior specification methods based on production
systems, recurrence equations, and Petri nets are defined and their expressive powers
are compared. Production system RDEM is extended to support non-determinism, and various
deterministic and non-deterministic production system interpreters are introduced and formally
compared in terms of their expressive power. It is shown that the parallel deterministic interpreter
has more expressive power than other interpreters including an OPS5-like interpreter.
Since it is also parallel, this makes the parallel deterministic interpreter a very attractive interpreter
for production systems.Information Systems Working Papers Serie
Aspects of dealing with imperfect data in temporal databases
In reality, some objects or concepts have properties with a time-variant or time-related nature. Modelling these kinds of objects or concepts in a (relational) database schema is possible, but time-variant and time-related attributes have an impact on the consistency of the entire database. Therefore, temporal database models have been proposed to deal with this. Time itself can be at the source of imprecision, vagueness and uncertainty, since existing time measuring devices are inherently imperfect. Accordingly, human beings manage time using temporal indications and temporal notions, which may contain imprecision, vagueness and uncertainty. However, the imperfection in human-used temporal indications is supported by human interpretation, whereas information systems need extraordinary support for this. Several proposals for dealing with such imperfections when modelling temporal aspects exist. Some of these proposals consider the basis of the system to be the conversion of the specificity of temporal notions between used temporal expressions. Other proposals consider the temporal indications in the used temporal expressions to be the source of imperfection. In this chapter, an overview is given, concerning the basic concepts and issues related to the modelling of time as such or in (relational) database models and the imperfections that may arise during or as a result of this modelling. Next to this, a novel and currently researched technique for handling some of these imperfections is presented
ON PERIODICITY IN TEMPORAL DATABASES
The issue of periodicity is generally understood to be a desirable property of temporal
data that should be supported by temporal database models and their query
languages. Nevertheless, there has so far not been any systematic examination of how
to incorporate this concept into a temporal DBMS. In this paper we describe two concepts
of periodicity, which we call strong periodicity and near periodicity, and discuss
how they capture formally two of the intuitive meanings of this term. We formally
compare the expressive power of these two concepts, relate them to existing temporal
query languages, and show how they can be incorporated into temporal relational
database query languages, such as the proposed temporal extension to SQL, in a clean
and straightforward manner.Information Systems Working Papers Serie