共查询到18条相似文献,搜索用时 82 毫秒
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半结构化数据查询重写 总被引:10,自引:1,他引:10
查询重写是数据库研究的一个基本问题,它和查询优化,数据仓库,信息集成,语义缓存等问题紧密相关,目前Internet上存在海量的半结构化数据,在信息集成过程中产生了大量半结构化视图,如何利用物化半结构化视图来重写用户查询,减少响应时间成为研究热点问题,上述问题本质上是NP问题,提出了一种半结构化查询重写的新方法,该方法在保证算法正确性和完备性的基础上,利用半结构化数据特点和查询子目标之间的关系,减少了指数空间的查询重写候选方案生成,理论分析表明,它极大地降低了算法的代价。 相似文献
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半结构化数据库没有固定的库模式,用户对其结构难以产生清晰的认识,从而无法有效地查询所需的内容.提出了一种基于本体的柔性查询,用户通过了解数据库本体语义信息而发出的查询不必遵循严格的数据库模式也能得出结果.由于在半结构化数据库上直接查找效率很低,故在其上生成描述结构模式的概念本体库.查询模块先在本体库上评估能否得出查询结果,再在数据库上执行查询.然而由于本体库可能是图的形式,其查询代价仍然很高,本质上是NP问题,进一步研究了将图转化为树的方法,并给出了相应的算法. 相似文献
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半结构化数据库中的交互式查询和搜索 总被引:1,自引:0,他引:1
魏定国 《计算机工程与应用》1998,34(9):6-8
文章提供了对半结构数据库进行交互查询、搜索的新模型。论述了有效关键字的搜索、查询结果的结构化总结和对逆向指针支持的重要性,并针对这些技术问题给出了初步解决方案。 相似文献
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随着XML的发展,Xquery变得越来越重要.本文介绍Xquery语言的特点和它的基本构造块--表达式,并从XML数据的特点出发,介绍XML的查询及其为查询优化提供一些技术参考。 相似文献
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在分析现有的半结构化数据的存储方式及存在的问题基础上,引入了小集合属性、集合属性、聚类集合、模板集合、父属性序列等概念,借助映射表达语言STORED,提出了一种基于数据挖掘的半结构化数据到结构化数据的模式抽取的方法。 相似文献
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XML查询语言XML-QL及其查询优化 总被引:6,自引:0,他引:6
从半结构化数据角度出发,通过一种XML查询语言-XML-QL介绍了XML文档查询过程,并为XML的查询优化提供了一种思路。 相似文献
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Dallan Quass Anand Rajaraman Jeffrey Ullman Jennifer Widom Yehoshua Sagiv 《Journal of Systems Integration》1997,7(3-4):381-407
Semistructured data has no absolute schema fixed in advance and its structure may be irregular or incomplete. Such data commonly arises in sources that do not impose a rigid structure (such as the World-Wide Web) and when data is combined from several heterogeneous sources. Data models and query languages designed for well structured data are inappropriate in such environments. Starting with a lightweight object model adopted for the TSIMMIS project at Stanford, in this paper we describe a query language and object repository designed specifically for semistructured data. Our language provides meaningful query results in cases where conventional models and languages do not: when some data is absent, when data does not have regular structure, when similar concepts are represented using different types, when heterogeneous sets are present, and when object structure is not fully known. This paper motivates the key concepts behind our approach, describes the language through a series of examples (a complete semantics is available in an accompanying technical report [23]), and describes the basic architecture and query processing strategy of the lightweight object repository we have developed. 相似文献
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基于XML的半结构数据的视图问题研究 总被引:1,自引:0,他引:1
1 引言数据库中的视图机制主要是根据用户或应用的需要对数据进行剪裁以增加数据库的灵活性。数据库的视图是适合某一特定用户或应用的数据库中部分数据的一种抽象。视图是依照视图声明语言(View Specification Language)来定义的,视图的声明是施加于源数据库(或等价的基数据库)上的。通常,数据库视图既可以是虚拟的(Virtual)、也可以是实际化的 相似文献
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分布式流查询是一种基于数据流的实时查询计算方法,近年来得到了广泛的关注和快速发展。综述了分布式流处理框架在实时关系型查询上取得的研究成果;对涉及分布式数据加载、分布式流计算框架、分布式流查询的产品进行了分析和比较;提出了基于Spark Streaming和Apache Kafka构建的分布式流查询模型,以并发加载多个文件源的形式,设计内存文件系统实现数据的快速加载,相较于基于Apache Flume的加载技术提速1倍以上。在Spark Streaming的基础上,实现了基于Spark SQL的分布式流查询接口,并提出了自行编码解析SQL语句的方法,实现了分布式查询。测试结果表明,在查询语句复杂的情况下,自行编码解析SQL的查询效率具有明显的优势。 相似文献
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《Journal of Computer and System Sciences》2002,64(3):655-693
Semistructured data occur in situations where information lacks a homogeneous structure and is incomplete. Yet, up to now the incompleteness of information has not been reflected by special features of query languages. Our goal is to investigate the principles of queries that allow for incomplete answers. We do not present, however, a concrete query language. Queries over classical structured data models contain a number of variables and constraints on these variables. An answer is a binding of the variables by elements of the database such that the constraints are satisfied. In the present paper, we loosen this concept in so far as we allow also answers that are partial; that is, not all variables in the query are bound by such an answer. Partial answers make it necessary to refine the model of query evaluation. The first modification relates to the satisfaction of constraints: in some circumstances we consider constraints involving unbound variables as satisfied. Second, in order to prevent a proliferation of answers, we only accept answers that are maximal in the sense that there are no assignments that bind more variables and satisfy the constraints of the query. Our model of query evaluation consists of two phases, a search phase and a filter phase. Semistructured databases are essentially labeled directed graphs. In the search phase, we use a query graph containing variables to match a maximal portion of the database graph. We investigate three different semantics for query graphs, which give rise to three variants of matching. For each variant, we provide algorithms and complexity results. In the filter phase, the maximal matchings resulting from the search phase are subjected to constraints, which may be weak or strong. Strong constraints require all their variables to be bound, while weak constraints do not. We describe a polynomial algorithm for evaluating a special type of queries with filter constraints, and assess the complexity of evaluating other queries for several kinds of constraints. In the final part, we investigate the containment problem for queries consisting only of search constraints under the different semantics. 相似文献
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基于XML的半结构数据查询语言研究 总被引:1,自引:0,他引:1
褚东升 《计算机工程与应用》2004,40(33):179-183
半结构数据管理的核心问题之一是数据的有效查询问题。文章重点分析、比较了两种基于XML的半结构查询语言,即XQL和XML-QL。在此基础上总结出了XML查询语言的基本需求,并对目前的XML查询语言提出了四点扩充建议。 相似文献
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This paper describes the theoretical framework and implementation of a database management system for storing and manipulating
diverse probability distributions of discrete random variables with finite domains, and associated information. A formal Semistructured
Probabilistic Object (SPO) data model and a Semistructured Probabilistic Query Algebra (SP-algebra) are proposed. The SP-algebra
supports standard database queries as well as some specific to probabilities, such as conditionalization and marginalization.
Thus, the Semistructured Probabilistic Database may be used as a backend to any application that involves the management of
large quantities of probabilistic information, such as building stochastic models. The implementation uses XML encoding of
SPOs to facilitate communication with diverse applications. The database management system has been implemented on top of
a relational DBMS. The translation of SP-algebra queries into relational queries are discussed here, and the results of initial
experiments evaluating the system are reported.
Work performed while a Ph.D. student at the University of Kentucky. 相似文献
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路径表达式作为XML数据查询语言的核心部分,关于它的计算方法的研究成果已有很多,然而针对路径表达式本身进行优化的研究却相对较少.提出了两种针对路径表达式的优化策略:路径缩短策略和补路径策略,从而提高了XML路径查询效率.路径缩短策略根据XML文档模式信息,将路径表达式查询长度缩短,从而简化查询本身以降低需要的查询代价;而补路径策略则试图使用代价更小的等价路径表达式来替换原始查询.经过对实验数据的分析,这两种优化策略对于绝大多数路径表达式查询可以应用,并可大幅度地改进路径表达式的查询性能. 相似文献