56 research outputs found
Queries with Guarded Negation (full version)
A well-established and fundamental insight in database theory is that
negation (also known as complementation) tends to make queries difficult to
process and difficult to reason about. Many basic problems are decidable and
admit practical algorithms in the case of unions of conjunctive queries, but
become difficult or even undecidable when queries are allowed to contain
negation. Inspired by recent results in finite model theory, we consider a
restricted form of negation, guarded negation. We introduce a fragment of SQL,
called GN-SQL, as well as a fragment of Datalog with stratified negation,
called GN-Datalog, that allow only guarded negation, and we show that these
query languages are computationally well behaved, in terms of testing query
containment, query evaluation, open-world query answering, and boundedness.
GN-SQL and GN-Datalog subsume a number of well known query languages and
constraint languages, such as unions of conjunctive queries, monadic Datalog,
and frontier-guarded tgds. In addition, an analysis of standard benchmark
workloads shows that most usage of negation in SQL in practice is guarded
negation
On relating CTL to Datalog
CTL is the dominant temporal specification language in practice mainly due to
the fact that it admits model checking in linear time. Logic programming and
the database query language Datalog are often used as an implementation
platform for logic languages. In this paper we present the exact relation
between CTL and Datalog and moreover we build on this relation and known
efficient algorithms for CTL to obtain efficient algorithms for fragments of
stratified Datalog. The contributions of this paper are: a) We embed CTL into
STD which is a proper fragment of stratified Datalog. Moreover we show that STD
expresses exactly CTL -- we prove that by embedding STD into CTL. Both
embeddings are linear. b) CTL can also be embedded to fragments of Datalog
without negation. We define a fragment of Datalog with the successor build-in
predicate that we call TDS and we embed CTL into TDS in linear time. We build
on the above relations to answer open problems of stratified Datalog. We prove
that query evaluation is linear and that containment and satisfiability
problems are both decidable. The results presented in this paper are the first
for fragments of stratified Datalog that are more general than those containing
only unary EDBs.Comment: 34 pages, 1 figure (file .eps
Monadic Datalog Containment on Trees
We show that the query containment problem for monadic datalog on finite
unranked labeled trees can be solved in 2-fold exponential time when (a)
considering unordered trees using the axes child and descendant, and when (b)
considering ordered trees using the axes firstchild, nextsibling, child, and
descendant. When omitting the descendant-axis, we obtain that in both cases the
problem is EXPTIME-complete.Comment: This article is the full version of an article published in the
proccedings of the 8th Alberto Mendelzon Workshop (AMW 2014
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
Linear Datalog and Bounded Path Duality of Relational Structures
In this paper we systematically investigate the connections between logics
with a finite number of variables, structures of bounded pathwidth, and linear
Datalog Programs. We prove that, in the context of Constraint Satisfaction
Problems, all these concepts correspond to different mathematical embodiments
of a unique robust notion that we call bounded path duality. We also study the
computational complexity implications of the notion of bounded path duality. We
show that every constraint satisfaction problem \csp(\best) with bounded path
duality is solvable in NL and that this notion explains in a uniform way all
families of CSPs known to be in NL. Finally, we use the results developed in
the paper to identify new problems in NL
Validation of schema mappings with nested queries
With the emergence of the Web and the wide use of XML for representing data, the ability to map not only flat relational but also nested data has become crucial. The design of schema mappings is a semi-automatic process. A human designer is needed to guide the process, choose among mapping candidates, and successively refine the mapping. The designer needs a way to figure out whether the mapping is what was intended. Our approach to mapping validation allows the designer to check whether the mapping satisfies certain desirable properties. In this paper, we focus on the validation of mappings between nested relational schemas, in which the mapping assertions are either inclusions or equalities of nested queries. We focus on the nested relational setting since most XML’s Document Type Definitions (DTDs) can be represented in this model. We perform the validation by reasoning on the schemas and mapping definition. We take into account the integrity constraints defined on both the source and target schema.Preprin
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