740,070 research outputs found
A SAT-based System for Consistent Query Answering
An inconsistent database is a database that violates one or more integrity
constraints, such as functional dependencies. Consistent Query Answering is a
rigorous and principled approach to the semantics of queries posed against
inconsistent databases. The consistent answers to a query on an inconsistent
database is the intersection of the answers to the query on every repair, i.e.,
on every consistent database that differs from the given inconsistent one in a
minimal way. Computing the consistent answers of a fixed conjunctive query on a
given inconsistent database can be a coNP-hard problem, even though every fixed
conjunctive query is efficiently computable on a given consistent database.
We designed, implemented, and evaluated CAvSAT, a SAT-based system for
consistent query answering. CAvSAT leverages a set of natural reductions from
the complement of consistent query answering to SAT and to Weighted MaxSAT. The
system is capable of handling unions of conjunctive queries and arbitrary
denial constraints, which include functional dependencies as a special case. We
report results from experiments evaluating CAvSAT on both synthetic and
real-world databases. These results provide evidence that a SAT-based approach
can give rise to a comprehensive and scalable system for consistent query
answering.Comment: 25 pages including appendix, to appear in the 22nd International
Conference on Theory and Applications of Satisfiability Testin
Differences between left- and right-handers in approach/avoidance motivation: influence of consistency of handedness measures
Hand preference is often viewed as a troublesome variable in psychological research, with left-handers routinely excluded from studies. Contrary to this, a body of evidence has shown hand preference to be a useful variable when examining human behavior. A recent review argues that the most effective way of using handedness as a variable, is a comparison between individuals who use their dominant hand for virtually all manual activities (consistent handers) versus those who use their other hand for at least one activity (inconsistent handers). The authors contend that researchers should only focus on degree of handedness rather than direction of preference (left versus right). However, we argue that the field suffers from a number of methodological and empirical issues. These include a lack of consensus in choice of cut-off point to divide consistent and inconsistent categories and importantly a paucity of data from left-handers. Consequentially, researchers predominantly compare inconsistent versus consistent right-handers, largely linked to memory, cognition and language. Other research on response style and personality measures shows robust direction of handedness effects. The present study examines both strength and direction of handedness on self-reported behavioral inhibition system (BIS) and behavioral activation system (BAS) scores, using evidence from a large (N = 689) dataset including more than 200 left-handers. There were degree of handedness effects on BIS and BAS-Fun Seeking, but effects are largely driven by differences between consistent left-handers and other groups. Choice of cut-off point substantively influenced results, and suggests that unless a suitable sample of left-handers is included, researchers clarify that their degree of handedness effects are applicable only to right-handers. We concur that strength of hand preference is an important variable but caution that differences related to consistency may not be identical in right and left-handers
Non-Filippov dynamics arising from the smoothing of nonsmooth systems, and its robustness to noise
Switch-like behaviour in dynamical systems may be modelled by highly
nonlinear functions, such as Hill functions or sigmoid functions, or
alternatively by piecewise-smooth functions, such as step functions. Consistent
modelling requires that piecewise-smooth and smooth dynamical systems have
similar dynamics, but the conditions for such similarity are not well
understood. Here we show that by smoothing out a piecewise-smooth system one
may obtain dynamics that is inconsistent with the accepted wisdom --- so-called
Filippov dynamics --- at a discontinuity, even in the piecewise-smooth limit.
By subjecting the system to white noise, we show that these discrepancies can
be understood in terms of potential wells that allow solutions to dwell at the
discontinuity for long times. Moreover we show that spurious dynamics will
revert to Filippov dynamics, with a small degree of stochasticity, when the
noise magnitude is sufficiently large compared to the order of smoothing. We
apply the results to a model of a dry-friction oscillator, where spurious
dynamics (inconsistent with Filippov's convention or with Coulomb's model of
friction) can account for different coefficients of static and kinetic
friction, but under sufficient noise the system reverts to dynamics consistent
with Filippov's convention (and with Coulomb-like friction).Comment: submitted to: Nonlinear Dynamic
Incremental Consistency Guarantees for Replicated Objects
Programming with replicated objects is difficult. Developers must face the
fundamental trade-off between consistency and performance head on, while
struggling with the complexity of distributed storage stacks. We introduce
Correctables, a novel abstraction that hides most of this complexity, allowing
developers to focus on the task of balancing consistency and performance. To
aid developers with this task, Correctables provide incremental consistency
guarantees, which capture successive refinements on the result of an ongoing
operation on a replicated object. In short, applications receive both a
preliminary---fast, possibly inconsistent---result, as well as a
final---consistent---result that arrives later.
We show how to leverage incremental consistency guarantees by speculating on
preliminary values, trading throughput and bandwidth for improved latency. We
experiment with two popular storage systems (Cassandra and ZooKeeper) and three
applications: a Twissandra-based microblogging service, an ad serving system,
and a ticket selling system. Our evaluation on the Amazon EC2 platform with
YCSB workloads A, B, and C shows that we can reduce the latency of strongly
consistent operations by up to 40% (from 100ms to 60ms) at little cost (10%
bandwidth increase, 6% throughput drop) in the ad system. Even if the
preliminary result is frequently inconsistent (25% of accesses), incremental
consistency incurs a bandwidth overhead of only 27%.Comment: 16 total pages, 12 figures. OSDI'16 (to appear
On the consistent use of linear demand systems if not all varieties are available
Linear demand formulations for price competition in horizontally differentiated products are sometimes used to compare situations where additional varieties become available, e.g. due to market entry of new firms. We derive a consistent demand system to analyze such situations and highlight potential problems that can arise from an inconsistent approach.
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