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We consider a scenario where nodes in a sensor network hold numeric items, and the task is to evaluate simple functions of the distributed data. In this note we present distributed protocols for computing the median with sublinear space and communication complexity per node. Specifically, we give a deterministic protocol for computing median with polylog complexity and a randomized protocol that computes an approximate median with polyloglog communication complexity per node. On the negative side, we observe that any deterministic protocol that counts the number of distinct data items must have linear complexity in the worst case. 相似文献
53.
S. Misbah Deen 《Data & Knowledge Engineering》1985,1(2):139-154
We propose here a .relational algebra capable of deducing tuples from a premise expressed in an extended relational form: it also supports user-defined and recursive functions in the form of parameterised views, and provides a facility for easier specification of queries. The paper shows the power of the language in dealing with problems such as ancestors, part explosions and connected tours.This language, called DEAL, is an enhanced version of the PRECI Algebraic Language (PAL) but presented here in a SQL-like syntax. 相似文献
54.
A partially specified nearest neighbor query is a nearest neighbor search in which only some of the possible keys are specified. An algorithm that uses k-d trees to perform such searching is described. The expected time complexity is O(N1-). where k is the total number of keys and j the number of keys specified in the query. Experimental results, which are consistent with the theoretical predictions, are also presented. 相似文献
55.
本文从试验控制的角度,论述了计算机可靠性工程的基本运转模式,通过PDCA循环的实施,使计算机的可靠性工程步入了良性发展的轨道.文中详细叙述了实施过程,对计算机可靠性工程实施方法进行了概括性总结. 相似文献
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Being decades of study, the usability of database systems have received more attention in recent years. Now it is especially able to explain missing objects in a query result, which is called “why-not” questions, and is the focus of concern. This paper studies the problem of answering whynot questions on KNN queries. In our real life, many users would like to use KNN queries to investigate the surrounding circumstances. Nevertheless, they often feel disappointed when finding the result not including their expected objects. In this paper, we use the query refinement approach to resolve the problem. Given the original KNN query and a set of missing objects as input, our algorithm offer a refined KNN query that includes the missing objects to the user. The experimental results demonstrate the efficiency of our proposed optimizations and algorithms. 相似文献
58.
Motivated by the continuing interest in the tree data model, we study the expressive power of downward navigational query languages on trees and chains. Basic navigational queries are built from the identity relation and edge relations using composition and union. We study the effects on relative expressiveness when we add transitive closure, projections, coprojections, intersection, and difference; this for Boolean queries and path queries on labeled and unlabeled structures. In all cases, we present the complete Hasse diagram. In particular, we establish, for each query language fragment that we study on trees, whether it is closed under difference and intersection. 相似文献
59.
M. Sanchiz F. Amadieu P.-V. Paubel A. Chevalier 《Behaviour & Information Technology》2020,39(10):1094-1109
ABSTRACT This study investigates how a search interface that displays users’ ultimate query (i.e. users’ current search goal) can cope with the age-related decrease of fluid abilities and support older users’ search behaviours. 30 young and 18 older adults completed 9 search problems with a regular web browser or with the experimental search interface. Results showed that older adults spent longer time on the search engine result pages, they needed more time to reformulate, and they had more difficulties exploring the search paths elaborated. Age-differences also appeared as soon as the beginning of the search. The support tool helped older users reformulate their queries more rapidly and elaborate more flexible search strategies at the beginning of the activity. Indeed, older adults who interacted with the support tool switched to the processing of a new search path more rapidly instead of exploiting their initial query (i.e. they visited fewer websites for the initial query produced and reformulated a query instead of keeping on conducting a deeper investigation of the search results provided in the search engine result page). Implications of these findings for the design of effective support tools for older users are discussed. 相似文献
60.
We consider two issues in polynomial-time exact learning of concepts using membership and equivalence queries: (1) errors or omissions in answers to membership queries, and (2) learning finite variants of concepts drawn from a learnable class.To study (1), we introduce two new kinds of membership queries: limited membership queries and malicious membership queries. Each is allowed to give incorrect responses on a maliciously chosen set of strings in the domain. Instead of answering correctly about a string, a limited membership query may give a special I don't know answer, while a malicious membership query may give the wrong answer. A new parameter Lis used to bound the length of an encoding of the set of strings that receive such incorrect answers. Equivalence queries are answered correctly, and learning algorithms are allowed time polynomial in the usual parameters and L. Any class of concepts learnable in polynomial time using equivalence and malicious membership queries is learnable in polynomial time using equivalence and limited membership queries; the converse is an open problem. For the classes of monotone monomials and monotone k-term DNF formulas, we present polynomial-time learning algorithms using limited membership queries alone. We present polynomial-time learning algorithms for the class of monotone DNF formulas using equivalence and limited membership queries, and using equivalence and malicious membership queries.To study (2), we consider classes of concepts that are polynomially closed under finite exceptions and a natural operation to add exception tables to a class of concepts. Applying this operation, we obtain the class of monotone DNF formulas with finite exceptions. We give a polynomial-time algorithm to learn the class of monotone DNF formulas with finite exceptions using equivalence and membership queries. We also give a general transformation showing that any class of concepts that is polynomially closed under finite exceptions and is learnable in polynomial time using standard membership and equivalence queries is also polynomial-time learnable using malicious membership and equivalence queries. Corollaries include the polynomial-time learnability of the following classes using malicious membership and equivalence queries: deterministic finite acceptors, boolean decision trees, and monotone DNF formulas with finite exceptions. 相似文献