Copyright (c) 2009 University of Pennsylvania All rights reserved. http://repository.upenn.edu/db_research Recent documents in Database Research Group (CIS) en-us Thu, 10 Dec 2009 03:22:00 PST 3600 http://repository.upenn.edu/db_research/46 http://repository.upenn.edu/db_research/46 Tue, 08 Dec 2009 08:47:39 PST We study containment and equivalence of (unions of) conjunctive queries on relations annotated with elements of a commutative semiring. Such relations and the semantics of positive relational queries on them were introduced in a recent paper as a generalization of set semantics, bag semantics, incomplete databases, and databases annotated with various kinds of provenance information. We obtain positive decidability results and complexity characterizations for databases with lineage, why-provenance, and provenance polynomial annotations, for both conjunctive queries and unions of conjunctive queries. At least one of these results is surprising given that provenance polynomial annotations seem "more expressive" than bag semantics and under the latter, containment of unions of conjunctive queries is known to be undecidable. The decision procedures rely on interesting variations on the notion of containment mappings. We also show that for any positive semiring (a very large class) and conjunctive queries without self-joins, equivalence is the same as isomorphism. Todd J. Green http://repository.upenn.edu/db_research/45 http://repository.upenn.edu/db_research/45 Tue, 08 Dec 2009 08:47:36 PST Exact query reformulation using views in positive relational languages is well understood, and has a variety of applications in query optimization and data sharing. Generalizations to larger fragments of the relational algebra (RA) --- specifically, support for the difference operator --- would increase the options available for query reformulation, and also apply to view adaptation (updating a materialized view in response to a modified view definition) and view maintenance. Unfortunately, most questions about queries become undecidable in the presence of difference/negation. We present a novel way of managing this difficulty via an excursion through a non-standard semantics, Z-relations, where tuples are annotated with positive or negative integers.We show that under Z-semantics RA queries have a normal form as a single difference of positive queries and this leads to the decidability of equivalence. In most real-world settings with difference, it is possible to convert the queries to this normal form. We give a sound and complete algorithm that explores all reformulations of an RA query (under Z-semantics) using a set of RA views, finitely bounding the search space with a simple and natural cost model. We investigate related complexity questions, and we also extend our results to queries with built-in predicates.Z-relations are interesting in their own right because they capture updates and data uniformly. However, our algorithm turns out to be sound and complete also for bag semantics, albeit necessarily only for a subclass of RA. This subclass turns out to be quite large and covers generously the applications of interest to us. We also show a subclass of RA where reformulation and evaluation under Z-semantics can be combined with duplicate elimination to obtain the answer under set semantics. Todd J. Green http://repository.upenn.edu/db_research/44 http://repository.upenn.edu/db_research/44 Mon, 06 Jul 2009 11:33:24 PDT We propose a mathematical framework, inspired by the Wireless HART specification, for modeling and analyzing multi-hop communication networks. The framework is designed for systems consisting of multiple control loops closed over a multi-hop communication network. We separate control, topology, routing, and scheduling and propose formal syntax and semantics for the dynamics of the composed system. The main technical contribution of the paper is an explicit translation of multi-hop control networks to switched systems. We describe a Mathematica notebook that automates the translation of multihop control networks to switched systems, and use this tool to show how techniques for analysis of switched systems can be used to address control and networking co-design challenges. Alur Rajeev http://repository.upenn.edu/db_research/43 http://repository.upenn.edu/db_research/43 Tue, 17 Mar 2009 06:44:31 PDT With the ultimate goal of extending the data integration paradigm and query processing capabilities to ad hoc wireless networks, sensors, and stream systems, we consider how to support communication between sets of nodes performing distributed joins in sensor networks. We develop a communication model that enables in-network join at a variety of locations, and which facilitates coordination among nodes in order to make optimization decisions. While we defer a discussion of the optimizer to future work, we experimentally compare a variety of strategies, including at-base and in-network joins. Results show significant performance gains versus prior work, as well as opportunities for optimization. Svilen Mihaylov http://repository.upenn.edu/db_research/42 http://repository.upenn.edu/db_research/42 Fri, 31 Oct 2008 06:01:21 PDT In many modern data management settings, data is queried from a central node or nodes, but is stored at remote sources. In such a setting it is common to perform "pushstyle" query processing, using multi-threaded pipelined hash joins and bushy query plans to compute parts of the query in parallel; to avoid idling, the CPU can switch between them as delays are encountered. This works well for simple select-project join queries, but increasingly, Web and integration applications require more complex queries with multiple joins and even nested subqueries. As we demonstrate in this paper, push-style execution of complex queries can be improved substantially via sideways information passing; push-style queries provide many opportunities for information passing that have not been studied in the past literature. We present adaptive information passing, a general runtime decision-making technique for reusing intermediate state from one query subresult to prune and reduce computation of other subresults. We develop two alternative schemes for performing adaptive information passing, which we study in several settings under a variety of workloads. Zachary G. Ives http://repository.upenn.edu/db_research/41 http://repository.upenn.edu/db_research/41 Fri, 11 Jul 2008 12:14:51 PDT We present a formal framework for capturing the provenance of data appearing in XQuery views of XML. Building on previous work on relations and their (positive) query languages, we decorate unordered XML with annotations from commutative semirings and show that these annotations suffice for a large positive fragment of XQuery applied to this data. In addition to tracking provenance metadata, the framework can be used to represent and process XML with repetitions, incomplete XML, and probabilistic XML, and provides a basis for enforcing access control policies in security applications.Each of these applications builds on our semantics for XQuery, which we present in several steps: we generalize the semantics of the Nested Relational Calculus (NRC) to handle semiring-annotated complex values, we extend it with a recursive type and structural recursion operator for trees, and we define a semantics for XQuery on annotated XML by translation into this calculus. John N. Foster http://repository.upenn.edu/db_research/40 http://repository.upenn.edu/db_research/40 Thu, 28 Jun 2007 13:26:49 PDT We consider the class of path-conjunctive queries and constraints (dependencies) defined over complex values with dictionaries. This class includes the relational conjunctive queries and embedded dependencies, as well as many interesting examples of complex value and oodb queries and integrity constraints. We show that some important classical results on containment, dependency implication, and chasing extend and generalize to this class. Val Tannen http://repository.upenn.edu/db_research/39 http://repository.upenn.edu/db_research/39 Thu, 28 Jun 2007 13:11:26 PDT The Web has become a major conduit to information repositories of all kinds. Today, more than 80% of information published on the Web is generated by underlying databases and this proportion keeps increasing. In some cases, database access is only granted through a Web gateway using forms as a query language and HTML as a display vehicle. In order to permit inter-operation (between Web sources and legacy databases or among Web sources themselves) there is a strong need for Web wrappers. Web wrappers share some of the characteristics of standard database wrappers but usually the underlying data sources offer very limited query capabilities and the struc- ture of the result (due to HTML shortcomings) might be loose and unstable. To overcome these problems, we divide the architecture of our Web wrappers into three components: (1) fetching the document, (2) extracting the information from its HTML formatting, and (3) mapping the information into a structure that can be used by applications (such as mediators). Fabien Azavant http://repository.upenn.edu/db_research/38 http://repository.upenn.edu/db_research/38 Thu, 28 Jun 2007 13:02:08 PDT This thesis examines the problems of performing structural transformations on databases involving complex data-structures and object-identities, and proposes an approach to specifying and implementing such transformations. We start by looking at various applications of such database transformations, and at some of the more significant work in these areas. In particular we will look at work on transformations in the area of database integration, which has been one of the major motivating areas for this work. We will also look at various notions of correctness that have been proposed for database transformations, and show that the utility of such notions is limited by the dependence of transformations on certain implicit database constraints. We draw attention to the limitations of existing work on transformations, and argue that there is a need for a more general formalism for reasoning about database transformations and constraints. We will also argue that, in order to ensure that database transformations are well-defined and meaningful, it is necessary to understand the information capacity of the data-models being transformed. To this end we give a thorough analysis of the information capacity of data-models supporting object identity, and will show that this is dependent on the operations supported by a query language for comparing object identities. We introduce a declarative language, WOL, based on Horn-clause logic, for specifying database transformations and constraints. We also propose a method of implementing transformations specified in this language, by manipulating their clauses into a normal form which can then be translated into an underlying database programming language. Finally we will present a number of optimizations and techniques necessary in order to build a practical implementation based on these proposals, and will discuss the results of some of the trials that were carried out using a prototype of such a system. Anthony S. Kosky http://repository.upenn.edu/db_research/37 http://repository.upenn.edu/db_research/37 Thu, 28 Jun 2007 12:28:36 PDT We define a general model for database schemas which is basically functional and supports specialisation relationships. Despite it's simplicity, our model is very general and expressive, so that database schemas and instances arising from a number of other data models can be translated into the model.We define and investigate a representation for the observations that can be made by querying a database system, and, in particular, look at which observations are valid for a particular database schema, and when one observation implies the observability of another. We will also look at the correspondence between the instances of a database schema and the observations that can be made for the database.We then go on to look at the problem of schema merging: we define an ordering on schemas representing their informational content and define the merge of a collection of schemas to be the least schema with the informational content of all the schemas being merged. However we establish that one cannot, in general, find a meaningful binary merging operator which is associative, though we would clearly require this of any such operator. We rectify this situation by relaxing our definition of schemas, defining a class of weak schemas over which we can construct a satisfactory concept of merges. Further we define a method of constructing a canonical proper schema with the same informational content as a weak schema whenever possible, thus giving us an adequate definition of the merge of a collection of proper schemas whenever such a merge can exist. In addition we show that, if the schemas we are merging are translations from some other data model, our merging process "respects" the original data model. Anthony S. Kosky