9,550 research outputs found
A survey of parallel execution strategies for transitive closure and logic programs
An important feature of database technology of the nineties is the use of parallelism for speeding up the execution of complex queries. This technology is being tested in several experimental database architectures and a few commercial systems for conventional select-project-join queries. In particular, hash-based fragmentation is used to distribute data to disks under the control of different processors in order to perform selections and joins in parallel. With the development of new query languages, and in particular with the definition of transitive closure queries and of more general logic programming queries, the new dimension of recursion has been added to query processing. Recursive queries are complex; at the same time, their regular structure is particularly suited for parallel execution, and parallelism may give a high efficiency gain. We survey the approaches to parallel execution of recursive queries that have been presented in the recent literature. We observe that research on parallel execution of recursive queries is separated into two distinct subareas, one focused on the transitive closure of Relational Algebra expressions, the other one focused on optimization of more general Datalog queries. Though the subareas seem radically different because of the approach and formalism used, they have many common features. This is not surprising, because most typical Datalog queries can be solved by means of the transitive closure of simple algebraic expressions. We first analyze the relationship between the transitive closure of expressions in Relational Algebra and Datalog programs. We then review sequential methods for evaluating transitive closure, distinguishing iterative and direct methods. We address the parallelization of these methods, by discussing various forms of parallelization. Data fragmentation plays an important role in obtaining parallel execution; we describe hash-based and semantic fragmentation. Finally, we consider Datalog queries, and present general methods for parallel rule execution; we recognize the similarities between these methods and the methods reviewed previously, when the former are applied to linear Datalog queries. We also provide a quantitative analysis that shows the impact of the initial data distribution on the performance of methods
Enumeration of Extractive Oracle Summaries
To analyze the limitations and the future directions of the extractive
summarization paradigm, this paper proposes an Integer Linear Programming (ILP)
formulation to obtain extractive oracle summaries in terms of ROUGE-N. We also
propose an algorithm that enumerates all of the oracle summaries for a set of
reference summaries to exploit F-measures that evaluate which system summaries
contain how many sentences that are extracted as an oracle summary. Our
experimental results obtained from Document Understanding Conference (DUC)
corpora demonstrated the following: (1) room still exists to improve the
performance of extractive summarization; (2) the F-measures derived from the
enumerated oracle summaries have significantly stronger correlations with human
judgment than those derived from single oracle summaries.Comment: 12 page
Efficient Color-Dressed Calculation of Virtual Corrections
With the advent of generalized unitarity and parametric integration
techniques, the construction of a generic Next-to-Leading Order Monte Carlo
becomes feasible. Such a generator will entail the treatment of QCD color in
the amplitudes. We extend the concept of color dressing to one-loop amplitudes,
resulting in the formulation of an explicit algorithmic solution for the
calculation of arbitrary scattering processes at Next-to-Leading order. The
resulting algorithm is of exponential complexity, that is the numerical
evaluation time of the virtual corrections grows by a constant multiplicative
factor as the number of external partons is increased. To study the properties
of the method, we calculate the virtual corrections to -gluon scattering.Comment: 48 pages, 23 figure
First steps in synthetic guarded domain theory: step-indexing in the topos of trees
We present the topos S of trees as a model of guarded recursion. We study the
internal dependently-typed higher-order logic of S and show that S models two
modal operators, on predicates and types, which serve as guards in recursive
definitions of terms, predicates, and types. In particular, we show how to
solve recursive type equations involving dependent types. We propose that the
internal logic of S provides the right setting for the synthetic construction
of abstract versions of step-indexed models of programming languages and
program logics. As an example, we show how to construct a model of a
programming language with higher-order store and recursive types entirely
inside the internal logic of S. Moreover, we give an axiomatic categorical
treatment of models of synthetic guarded domain theory and prove that, for any
complete Heyting algebra A with a well-founded basis, the topos of sheaves over
A forms a model of synthetic guarded domain theory, generalizing the results
for S
Meromorphic solutions of recurrence relations and DRA method for multicomponent master integrals
We formulate a method to find the meromorphic solutions of higher-order
recurrence relations in the form of the sum over poles with coefficients
defined recursively. Several explicit examples of the application of this
technique are given. The main advantage of the described approach is that the
analytical properties of the solutions are very clear (the position of poles is
explicit, the behavior at infinity can be easily determined). These are exactly
the properties that are required for the application of the multiloop
calculation method based on dimensional recurrence relations and analyticity
(the DRA method).Comment: 20 pages, minor change
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