Database Theory Column Report on PODS 2021

The 40th edition of the ACM SIGMOD-SIGACT-SIGAI Symposium on Principles of Database Systems (PODS) was held from June 20 to June 25, 2021, in Xi'an, Shaanxi, China. Due to the COVID-19 pandemic, it was organized in hybrid mode, with a local event primarily targeting the Chinese data management community and as a virtual (on- line) conference for the international community. As in previous years, the symposium was held jointly with the ACM International Conference on Management of Data (SIGMOD). PODS focuses on theoretical aspects of data management systems, and the co-location with SIGMOD stimulates interaction between theory-oriented and system-oriented research.

RECENT RESULTS ON CARDINALITY ESTIMATION AND INFORMATION THEORETIC INEQUALITIES

I would like to dedicate this little exposition to Prof. Phan Dinh Dieu, one of the giants and pioneers of Mathematics in Computer Science in Vietnam. In the past 15 years or so, new and exciting connections between fundamental problems in database theory and information theory have emerged. There are several angles one can take to describe this connection. This paper takes one such angle, influenced by the author's own bias and research results. In particular, we will describe how the cardinality estimation problem -- a corner-stone problem for query optimizers -- is deeply connected to information theoretic inequalities. Furthermore, we explain how inequalities can also be used to derive a couple of classic geometric inequalities such as the Loomis-Whitney inequality. A purpose of the article is to introduce the reader to these new connections, where theory and practice meet in a wonderful way. Another objective is to point the reader to a research area with many new open questions.

Ensuring Data Integrity in Databases with the Universal Basis of Relations

The objective of the paper was to reveal the main techniques and means of ensuring the integrity of data and persistent stored database modules implemented in accordance with the recommendations of the Clark–Wilson model as a methodological basis for building a system that ensures integrity. The considered database was built according to the schema with the universal basis of relations. The mechanisms developed in the process of researching the problem of ensuring the integrity of the data and programs of such a database were based on the provisions of the relational database theory, the Row Level Security technology, the potential of the modern blockchain model, and the capabilities of the database management system on the platform of which databases with the universal basis of relations are implemented. The implementation of the proposed techniques and means, controlling the integrity of the database of stored elements, prevents their unauthorized modification by authorized subjects and hinders the introduction of changes by unauthorized subjects. As a result, the stored data and programs remain correct, unaltered, undistorted, and preserved. This means that databases built based on a schema with the universal basis of relations and supported by such mechanisms are protected in terms of integrity.

METHOD OF MODELING OF A SOCIAL PROFILE USING BIG DATA STRUCTURE TRANSFORMATION OPTIMIZATION

The object of the research are methods and algorithms of optimizing of the Big Data transformation to build a social profile model, the subject of the research are methods of constructing of a social profile. For decision-making person, the problem of scientific methodological and instrumental re-equipment is relevant for the effective fulfillment of a set of managerial tasks and confronting of fundamentally new challenges and threats in society. This task is directly related to the problem of building of a model of the social profile of both the individual and the social group as a whole. Therefore, the problem of optimizing of methods of constructing of a mathematical model of a social profile is certainly relevant. During the research, methods of the mathematical apparatus of graph theory, database theory and the concept of non-relational data stores, Big Data technology, text analytics technologies, parallel data processing methods, methods of neural networks' using, methods of multimedia data analyzing were used. These methods were integrated into the general method, called the method of increasing of the efficiency of constructing of a mathematical model of a social profile. The proposed method improves the adequacy of the social profile model, which will significantly improve and simplify the functioning of information systems for decision-making based on knowledge of the social advantages of certain social groups, which will allow dynamic correction of their behavior. The obtained results of testing the method make it possible to consider it as an effective tool for obtaining of an objective information model of a social portrait of a social group. This is because the correctness of setting and solving of the problem ensured that adequate results were obtained. Unlike the existing ones, the proposed modeling method, which uses an oriented graph, allows to improve significantly the quality and adequacy of this process. Further research should be directed towards the implementation of proposed theoretical developments in real decision-making systems. This will increase the weight of automated decision-making systems for social climate analysis.

Holding a Conference Online and Live due to Covid-19

The joint EDBT/ICDT conference (International Conference on Extending Database Technology / International Conference on Database Theory) is a well established conference series on data management, with annual meetings in the second half of March that attract 250 to 300 delegates. Three weeks before EDBT/ICDT 2020 was planned to take place in Copenhagen, the rapidly developing Covid-19 pandemic led to the decision to cancel the face-to-face event. In the interest of the research community, it was decided to move the conference online while trying to preserve as much of the real-life experience as possible. As far as we know, we are one of the first conferences that moved to a fully synchronous online experience due to the COVID- 19 outbreak. By fully synchronous, we mean that participants jointly listened to presentations, had live Q&A, and attended other live events associated with the conference. In this report, we share our decisions, experiences, and lessons learned.

All-Instances Restricted Chase Termination for Linear TGDs

The chase procedure is a fundamental algorithmic tool in database theory with a variety of applications. A key problem concerning the chase procedure is all-instances chase termination: for a given set of tuple-generating dependencies (TGDs), is it the case that the chase terminates for every input database? In view of the fact that this problem is, in general, undecidable, it is natural to ask whether well-behaved classes of TGDs, introduced in different contexts, ensure decidability. It has been recently shown that the problem is decidable for the restricted (a.k.a. standard) version of the chase, and linear TGDs, a prominent class of TGDs that has been introduced in the context of ontological query answering, under the assumption that only one atom appears in TGD-heads. We provide an alternative proof for this result based on Monadic Second-Order Logic, which we believe is simpler that the ones obtained from the literature.

Semantic Width and the Fixed-Parameter Tractability of Constraint Satisfaction Problems

Constraint satisfaction problems (CSPs) are an important formal framework for the uniform treatment of various prominent AI tasks, e.g., coloring or scheduling problems. Solving CSPs is, in general, known to be NP-complete and fixed-parameter intractable when parameterized by their constraint scopes. We give a characterization of those classes of CSPs for which the problem becomes fixed-parameter tractable. Our characterization significantly increases the utility of the CSP framework by making it possible to decide the fixed-parameter tractability of problems via their CSP formulations. We further extend our characterization to the evaluation of unions of conjunctive queries, a fundamental problem in databases. Furthermore, we provide some new insight on the frontier of PTIME solvability of CSPs. In particular, we observe that bounded fractional hypertree width is more general than bounded hypertree width only for classes that exhibit a certain type of exponential growth. The presented work resolves a long-standing open problem and yields powerful new tools for complexity research in AI and database theory.