A Dichotomy on the Complexity of Consistent Query Answering for Atoms with Simple Keys

We study the problem of consistent query answering under primary key violations. In this setting, the relations in a database violate the key constraints and we are interested in maximal subsets of the database that satisfy the constraints, which we call repairs. For a boolean query Q, the problem CERTAINTY(Q) asks whether every such repair satisfies the query or not; the problem is known to be always in coNP for conjunctive queries. However, there are queries for which it can be solved in polynomial time. It has been conjectured that there exists a dichotomy on the complexity of CERTAINTY(Q) for conjunctive queries: it is either in PTIME or coNP-complete. In this paper, we prove that the conjecture is indeed true for the case of conjunctive queries without self-joins, where each atom has as a key either a single attribute (simple key) or all attributes of the atom

The Design of Arbitrage-Free Data Pricing Schemes

Motivated by a growing market that involves buying and selling data over the web, we study pricing schemes that assign value to queries issued over a database. Previous work studied pricing mechanisms that compute the price of a query by extending a data seller's explicit prices on certain queries, or investigated the properties that a pricing function should exhibit without detailing a generic construction. In this work, we present a formal framework for pricing queries over data that allows the construction of general families of pricing functions, with the main goal of avoiding arbitrage. We consider two types of pricing schemes: instance-independent schemes, where the price depends only on the structure of the query, and answer-dependent schemes, where the price also depends on the query output. Our main result is a complete characterization of the structure of pricing functions in both settings, by relating it to properties of a function over a lattice. We use our characterization, together with information-theoretic methods, to construct a variety of arbitrage-free pricing functions. Finally, we discuss various tradeoffs in the design space and present techniques for efficient computation of the proposed pricing functions.Comment: full pape

Worst-Case Optimal Algorithms for Parallel Query Processing

In this paper, we study the communication complexity for the problem of computing a conjunctive query on a large database in a parallel setting with $p$ servers. In contrast to previous work, where upper and lower bounds on the communication were specified for particular structures of data (either data without skew, or data with specific types of skew), in this work we focus on worst-case analysis of the communication cost. The goal is to find worst-case optimal parallel algorithms, similar to the work of [18] for sequential algorithms. We first show that for a single round we can obtain an optimal worst-case algorithm. The optimal load for a conjunctive query $q$ when all relations have size equal to $M$ is $O(M/p^{1/\psi^*})$, where $\psi^*$ is a new query-related quantity called the edge quasi-packing number, which is different from both the edge packing number and edge cover number of the query hypergraph. For multiple rounds, we present algorithms that are optimal for several classes of queries. Finally, we show a surprising connection to the external memory model, which allows us to translate parallel algorithms to external memory algorithms. This technique allows us to recover (within a polylogarithmic factor) several recent results on the I/O complexity for computing join queries, and also obtain optimal algorithms for other classes of queries

Communication Steps for Parallel Query Processing

We consider the problem of computing a relational query $q$ on a large input database of size $n$, using a large number $p$ of servers. The computation is performed in rounds, and each server can receive only $O(n/p^{1-\varepsilon})$ bits of data, where $\varepsilon \in [0,1]$ is a parameter that controls replication. We examine how many global communication steps are needed to compute $q$. We establish both lower and upper bounds, in two settings. For a single round of communication, we give lower bounds in the strongest possible model, where arbitrary bits may be exchanged; we show that any algorithm requires $\varepsilon \geq 1-1/\tau^*$, where $\tau^*$ is the fractional vertex cover of the hypergraph of $q$. We also give an algorithm that matches the lower bound for a specific class of databases. For multiple rounds of communication, we present lower bounds in a model where routing decisions for a tuple are tuple-based. We show that for the class of tree-like queries there exists a tradeoff between the number of rounds and the space exponent $\varepsilon$. The lower bounds for multiple rounds are the first of their kind. Our results also imply that transitive closure cannot be computed in O(1) rounds of communication

Testing for Structural Breaks and other forms of Non-stationarity: a Misspecification Perspective

In the 1980s and 1990s the issue of non-stationarity in economic time series has been in the context of unit roots vs. mean trends in AR(p) models. More recently this perspective has been extended to include structural breaks. In this paper we take a much broader perspective by viewing the problem as one of misspecification testing: assessing the stationarity of the underlying process. The proposed misspecification testing procedure relies on resampling techniques to enhance the informational content of the observed data in an attempt to capture heterogeneity `locally' using rolling window estimators of the primary moments of the stochastic process. The effectiveness of the testing procedure is assessed using extensive Monte Carlo simulationsMaximum Entropy Bootstrap, Non-Stationarity

Teaching and learning of language structures and conventions in the intermediate phase English home language classroom: a comparative study of a public and an independent school in South Africa

Research report submitted in partial fulfilment of the requirements of the degree Masters in applied language and literacy education in the division of languages, literacies and literatures school of education at the University of the Witwatersrand June 2017This study explored and compared how language structures and conventions were taught in the intermediate phase English Home Language classroom at a public and at an independent school in central Johannesburg, South Africa. The aim was to identify challenges and strategies in an attempt at promoting pedagogies that enhance the learning of language structures and conventions. Specific themes related to the teaching and learning of language structures and conventions have been highlighted as the main point of departure in the study. Such themes include: a synthesis of the curriculum applied at public and independent schools; theoretical viewpoints in the teaching and learning of language structures and conventions, highlighting challenges experienced in the EHL classroom; and lastly, the teaching strategies used in overcoming the challenges. A qualitative research approach was adopted, using a case study as the research design. Most case studies allow space for generalisations to be made, however, in this study I was limited to making such generalisations because my study is considered a minor study. The research methods consisted of: biographical questionnaires; 8 teacher interviews (Grades 4 – 6); and 5 classroom observations (only Grade 6) per school which were audio-taped. The findings suggested that the most common teaching approach used throughout the lessons in both schools was the text-based approach, which is advocated in the CAPS (2011). The classroom observations conducted at the public school illustrated that the Audio-lingual Method of language teaching was used, as opposed to CLT which was evident in the classroom observations at the independent school. Lessons in the public school were primarily teacher-centered, where the teacher engaged in giving instructions, providing explanations and eliciting responses. The type of questions asked consisted of lower level thinking (literal). Learners in the independent school, however, showed more enthusiasm towards participating in classroom discussions and answering questions, thus creating a learner-centered culture. It is evident that the independent school is more flexible in selecting the language components that need to be covered within a specific period. Based on the interview sessions, the teachers at the independent school mention that it is more important for them to cover a minimum amount of work in-depth rather than covering everything and not having learners understand what they are learning. As a future recommendation, a similar study can be conducted comparing an urban public school and a rural public school that apply the CAPS. This study adds to the understanding of how language structures and conventions are taught in the intermediate phase EHL classroom in the South African context. By encouraging and promoting the teaching of grammar, we are able to identify approaches best suited for quality learning to take place.MT 201