18 research outputs found
A New Client-Server Architecture for Distributed Query Processing
This paper presents the idea of "tuple bit-vectors" for distributed query processing. Using tuple bit-vectors, a new two-way semijoin operator called 2SJ++ that enhances the semijoin with an essentially "free" backward reduction capability is proposed. We explore in detail the benefits and costs of 2SJ++ compared with other semijoin variants, and its effect on distributed query processing performance. We then focus on one particular distributed query processing algorithm, called the "one-shot" algorithm. We modify the one-shot algorithm by using 2SJ++ and demonstrate the improvements achieved in network transmission cost compared with the original one-shot technique. We use this improvement to demonstrate that equipped with the 2SJ++ technique, one can improve the performance of distributed query processing algorithms significantly without adding much complexity to the algorithms
Execution strategies for SQL subqueries
Optimizing SQL subqueries has been an active area in database research and the database industry throughout the last decades. Pre-vious work has already identified some approaches to efficiently execute relational subqueries. For satisfactory performance, proper choice of subquery execution strategies becomes even more essen-tial today with the increase in decision support systems and auto-matically generated SQL, e.g., with ad-hoc reporting tools. This goes hand in hand with increasing query complexity and growing data volumes – which all pose challenges for an industrial-strength query optimizer. This current paper explores the basic building blocks that Microsoft SQL Server utilizes to optimize and execute relational subqueries. We start with indispensable prerequisites such as detection and removal of correlations for subqueries. We identify a full spectrum of fundamental subquery execution strategies such as forward and reverse lookup as well as set-based approaches, explain the different execution strategies for subqueries implemented in SQL Server, and relate them to the current state of the art. To the best of our knowl-edge, several strategies discussed in this paper have not been pub-lished before. An experimental evaluation complements the paper. It quantifies the performance characteristics of the different approaches and shows that indeed alternative execution strategies are needed in different circumstances, which make a cost-based query optimizer indispen-sable for adequate query performance
Finding intersection models: From chordal to Helly circular-arc graphs
Every chordal graph G admits a representation as the intersection graph of a family of subtrees of a tree. A classic way of finding such an intersection model is to look for a maximum spanning tree of the valuated clique graph of G. Similar techniques have been applied to find intersection models of chordal graph subclasses as interval graphs and path graphs. In this work, we extend those methods to be applied beyond chordal graphs: we prove that a graph G can be represented as the intersection of a Helly separating family of graphs belonging to a given class if and only if there exists a spanning subgraph of the clique graph of G satisfying a particular condition. Moreover, such a spanning subgraph is characterized by its weight in the valuated clique graph of G. The specific case of Helly circular-arc graphs is treated. We show that the canonical intersection models of those graphs correspond to the maximum spanning cycles of the valuated clique graph.Facultad de Ciencias Exacta
Bloom Filters in Adversarial Environments
Many efficient data structures use randomness, allowing them to improve upon
deterministic ones. Usually, their efficiency and correctness are analyzed
using probabilistic tools under the assumption that the inputs and queries are
independent of the internal randomness of the data structure. In this work, we
consider data structures in a more robust model, which we call the adversarial
model. Roughly speaking, this model allows an adversary to choose inputs and
queries adaptively according to previous responses. Specifically, we consider a
data structure known as "Bloom filter" and prove a tight connection between
Bloom filters in this model and cryptography.
A Bloom filter represents a set of elements approximately, by using fewer
bits than a precise representation. The price for succinctness is allowing some
errors: for any it should always answer `Yes', and for any it should answer `Yes' only with small probability.
In the adversarial model, we consider both efficient adversaries (that run in
polynomial time) and computationally unbounded adversaries that are only
bounded in the number of queries they can make. For computationally bounded
adversaries, we show that non-trivial (memory-wise) Bloom filters exist if and
only if one-way functions exist. For unbounded adversaries we show that there
exists a Bloom filter for sets of size and error , that is
secure against queries and uses only
bits of memory. In comparison, is the best
possible under a non-adaptive adversary
On the Complexity of Core, Kernel, and Bargaining Set
Coalitional games are mathematical models suited to analyze scenarios where
players can collaborate by forming coalitions in order to obtain higher worths
than by acting in isolation. A fundamental problem for coalitional games is to
single out the most desirable outcomes in terms of appropriate notions of worth
distributions, which are usually called solution concepts. Motivated by the
fact that decisions taken by realistic players cannot involve unbounded
resources, recent computer science literature reconsidered the definition of
such concepts by advocating the relevance of assessing the amount of resources
needed for their computation in terms of their computational complexity. By
following this avenue of research, the paper provides a complete picture of the
complexity issues arising with three prominent solution concepts for
coalitional games with transferable utility, namely, the core, the kernel, and
the bargaining set, whenever the game worth-function is represented in some
reasonable compact form (otherwise, if the worths of all coalitions are
explicitly listed, the input sizes are so large that complexity problems
are---artificially---trivial). The starting investigation point is the setting
of graph games, about which various open questions were stated in the
literature. The paper gives an answer to these questions, and in addition
provides new insights on the setting, by characterizing the computational
complexity of the three concepts in some relevant generalizations and
specializations.Comment: 30 pages, 6 figure
A join-based hybrid parameter for constraint satisfaction
We propose joinwidth, a new complexity parameter for the Constraint Satisfaction Problem (CSP). The definition of joinwidth is based on the arrangement of basic operations on relations (joins, projections, and pruning), which inherently reflects the steps required to solve the instance. We use joinwidth to obtain polynomial-time algorithms (if a corresponding decomposition is provided in the input) as well as fixed-parameter algorithms (if no such decomposition is provided) for solving the CSP.
Joinwidth is a hybrid parameter, as it takes both the graphical structure as well as the constraint relations that appear in the instance into account. It has, therefore, the potential to capture larger classes of tractable instances than purely structural parameters like hypertree width and the more general fractional hypertree width (fhtw). Indeed, we show that any class of instances of bounded fhtw also has bounded joinwidth, and that there exist classes of instances of bounded joinwidth and unbounded fhtw, so bounded joinwidth properly generalizes bounded fhtw. We further show that bounded joinwidth also properly generalizes several other known hybrid restrictions, such as fhtw with degree constraints and functional dependencies. In this sense, bounded joinwidth can be seen as a unifying principle that explains the tractability of several seemingly unrelated classes of CSP instances
Tree Projections and Constraint Optimization Problems: Fixed-Parameter Tractability and Parallel Algorithms
Tree projections provide a unifying framework to deal with most structural
decomposition methods of constraint satisfaction problems (CSPs). Within this
framework, a CSP instance is decomposed into a number of sub-problems, called
views, whose solutions are either already available or can be computed
efficiently. The goal is to arrange portions of these views in a tree-like
structure, called tree projection, which determines an efficiently solvable CSP
instance equivalent to the original one. Deciding whether a tree projection
exists is NP-hard. Solution methods have therefore been proposed in the
literature that do not require a tree projection to be given, and that either
correctly decide whether the given CSP instance is satisfiable, or return that
a tree projection actually does not exist. These approaches had not been
generalized so far on CSP extensions for optimization problems, where the goal
is to compute a solution of maximum value/minimum cost. The paper fills the
gap, by exhibiting a fixed-parameter polynomial-time algorithm that either
disproves the existence of tree projections or computes an optimal solution,
with the parameter being the size of the expression of the objective function
to be optimized over all possible solutions (and not the size of the whole
constraint formula, used in related works). Tractability results are also
established for the problem of returning the best K solutions. Finally,
parallel algorithms for such optimization problems are proposed and analyzed.
Given that the classes of acyclic hypergraphs, hypergraphs of bounded
treewidth, and hypergraphs of bounded generalized hypertree width are all
covered as special cases of the tree projection framework, the results in this
paper directly apply to these classes. These classes are extensively considered
in the CSP setting, as well as in conjunctive database query evaluation and
optimization