5,104 research outputs found
Finite Open-World Query Answering with Number Restrictions (Extended Version)
Open-world query answering is the problem of deciding, given a set of facts,
conjunction of constraints, and query, whether the facts and constraints imply
the query. This amounts to reasoning over all instances that include the facts
and satisfy the constraints. We study finite open-world query answering (FQA),
which assumes that the underlying world is finite and thus only considers the
finite completions of the instance. The major known decidable cases of FQA
derive from the following: the guarded fragment of first-order logic, which can
express referential constraints (data in one place points to data in another)
but cannot express number restrictions such as functional dependencies; and the
guarded fragment with number restrictions but on a signature of arity only two.
In this paper, we give the first decidability results for FQA that combine both
referential constraints and number restrictions for arbitrary signatures: we
show that, for unary inclusion dependencies and functional dependencies, the
finiteness assumption of FQA can be lifted up to taking the finite implication
closure of the dependencies. Our result relies on new techniques to construct
finite universal models of such constraints, for any bound on the maximal query
size.Comment: 59 pages. To appear in LICS 2015. Extended version including proof
When Can We Answer Queries Using Result-Bounded Data Interfaces?
We consider answering queries on data available through access methods, that
provide lookup access to the tuples matching a given binding. Such interfaces
are common on the Web; further, they often have bounds on how many results they
can return, e.g., because of pagination or rate limits. We thus study
result-bounded methods, which may return only a limited number of tuples. We
study how to decide if a query is answerable using result-bounded methods,
i.e., how to compute a plan that returns all answers to the query using the
methods, assuming that the underlying data satisfies some integrity
constraints. We first show how to reduce answerability to a query containment
problem with constraints. Second, we show "schema simplification" theorems
describing when and how result bounded services can be used. Finally, we use
these theorems to give decidability and complexity results about answerability
for common constraint classes.Comment: 65 pages; journal version of the PODS'18 paper arXiv:1706.0793
When Can We Answer Queries Using Result-Bounded Data Interfaces?
We consider answering queries where the underlying data is available only
over limited interfaces which provide lookup access to the tuples matching a
given binding, but possibly restricting the number of output tuples returned.
Interfaces imposing such "result bounds" are common in accessing data via the
web. Given a query over a set of relations as well as some integrity
constraints that relate the queried relations to the data sources, we examine
the problem of deciding if the query is answerable over the interfaces; that
is, whether there exists a plan that returns all answers to the query, assuming
the source data satisfies the integrity constraints.
The first component of our analysis of answerability is a reduction to a
query containment problem with constraints. The second component is a set of
"schema simplification" theorems capturing limitations on how interfaces with
result bounds can be useful to obtain complete answers to queries. These
results also help to show decidability for the containment problem that
captures answerability, for many classes of constraints. The final component in
our analysis of answerability is a "linearization" method, showing that query
containment with certain guarded dependencies -- including those that emerge
from answerability problems -- can be reduced to query containment for a
well-behaved class of linear dependencies. Putting these components together,
we get a detailed picture of how to check answerability over result-bounded
services.Comment: 45 pages, 2 tables, 43 references. Complete version with proofs of
the PODS'18 paper. The main text of this paper is almost identical to the
PODS'18 except that we have fixed some small mistakes. Relative to the
earlier arXiv version, many errors were corrected, and some terminology has
change
Detecting Changes in Pressure Using a Graphene Field Effect Transistor
Graphene is a single, atomic layer, hexagonal lattice with useful electrical properties. Discovered as a stable isolated sheet in the early 2000s, graphene field effect transistors (GFET) are an effective way to detect small changes in electrical activity. When an electrolytic fluid is placed on a GFET, a double layer capacitor can develop at the interface between the fluid and graphene. Surprisingly, this interface is sensitive to barometric pressure, making GFETs a viable device for measuring pressure changes. In this work we built a pressure vessel and placed GFETs inside to test the performance limits of graphene based on its environment
Goal-Driven Query Answering for Existential Rules with Equality
Inspired by the magic sets for Datalog, we present a novel goal-driven
approach for answering queries over terminating existential rules with equality
(aka TGDs and EGDs). Our technique improves the performance of query answering
by pruning the consequences that are not relevant for the query. This is
challenging in our setting because equalities can potentially affect all
predicates in a dataset. We address this problem by combining the existing
singularization technique with two new ingredients: an algorithm for
identifying the rules relevant to a query and a new magic sets algorithm. We
show empirically that our technique can significantly improve the performance
of query answering, and that it can mean the difference between answering a
query in a few seconds or not being able to process the query at all
Model Checking Markov Chains Against Unambiguous Buchi Automata
We give a polynomial-time algorithm for model checking finite Markov chains
against omega-regular specifications given as unambiguous Buchi automata
- …