Victor Vianu

Invited articles section foreword

This issue inaugurates the Invited Articles section of JACM. Each year, JACM invites a small number of articles to appear in the journal based on the recommendation of the Program Committees of several major conferences in Computer Science. All invited articles are reviewed according to the standard JACM refereeing process. In this issue, the Invited Articles section is comprised of the article “Epistemic Privacy” by Alexandre Evfimievski, Ronald Fagin and David Woodruff, selected from the 27th ACM SIGMOD-SIGACT-SIGART Symposium on Principles of Database Systems (PODS), held in Vancouver, Canada, June 9–11, 2008. I thank the Program Committee of PODS 2008 and the PC Chair, Maurizio Lenzerini, for their help in selecting this invited paper.

Datalog extensions for database queries and updates

Deterministic and non-deterministic extensions of Datalog with fixpoint semantics are proposed, and their expressive power characterized. It is argued that fixpoint semantics provides an elegant way to overcome the limited expressive power available with purely declarative semantics. The Datalog extensions range from complete languages to languages capturing interesting complexity classes of queries and updates: PTIME and PSPACE in the non-deterministic case, and the fixpoint queries and while queries in the deterministic case. The connection between the Datalog extensions and explicitly procedural languages, as well as fixpoint extensions of first-order logic, is also investigated. In particular, a new family of non-deterministic fixpoint extensions of first-order logic is considered.

DTD inference for views of XML data

We study the inference of Data Type Definitions (DTDs) for views of XML data, using an abstraction that focuses on document content structure. The views are defined by a query language that produces a list of documents selected from one or more input sources. The selection conditions involve vertical and horizontal navigation, thus querying explicitly the order present in input documents. We point several strong limitations in the descriptive ability of current DTDs and the need for extending them with (i) a subtyping mechanism and (ii) a more powerful specification mechanism than regular languages, such as context-free languages. With these extensions, we show that one can always infer tight DTDs, that precisely characterize a selection view on sources satisfying given DTDs. We also show important special cases where one can infer a tight DTD without requiring extension (ii). Finally we consider related problems such as verifying conformance of a view definition with a predefined DTD. Extensions to more powerful views that construct complex documents are also briefly discussed.

Generic Computation and its complexity

There is a fundamental mismatch between the hardness of database queries and their Turing complexity. For example, the even query on a set has low Turing complexity but is by all accounts a hard query. The mismatch is due to the abstract, genen’c nature of database computation: data items which are indistinguishable by logical properties are treated uniformly. The issues specific to generic computation are obscured in the Turing model. Two models of generic computation are pr~ posed. They are extensions of Turing Machines with a relational store. The machines differ in the interaction between the Turing component and the relational store. The first, simpler machine, allows limited communication with the relational store. However, it is not complete. Nonetheless, it subsumes many query languages which have emerged as central, including the jixpoint and while queries. We prove a normal form for the machine, which provides a key technical tool. The normal form specialized to the while queries allows resolving the open problem of the relation of jizpoint and while: they are equivalent iff PTIME = PSPACE. The second machine allows extended communication between the relational store and the Turing component, and is complete. The machine involves parallelism. Based on it, we define complexity measures for generic mappings. We focus on notions of polynomial time and space complexit y for generic mappings, and show their robustness. The results point to a trade-off between complexity and computing with an abstract interface. *Work supported in part by an INRIA-NSF cooperation grant, by the French Ministry of Research under grant PRC-BD3 and the Nationaf Science Foundation under grants IR1-881607S and lNT-881 7874. Work performed in part while the second author wss visiting lNRIA. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise , or to republish, requires a fee and/or specific permission. Victor Vianu

Finite state machines for strings over infinite alphabets

Motivated by formal models recently proposed in the context of XML, we study automata and logics on strings over infinite alphabets. These are conservative extensions of classical automata and logics defining the regular languages on finite alphabets. Specifically, we consider register and pebble automata, and extensions of first-order logic and monadic second-order logic. For each type of automaton we consider one-way and two-way variants, as well as deterministic, nondeterministic, and alternating control. We investigate the expressiveness and complexity of the automata and their connection to the logics, as well as standard decision problems. Some of our results answer open questions of Kaminski and Francez on register automata.

Regular path queries with constraints

The evaluation of path expression queries on semi-structured data in a distributed asynchronous environment is considered. The focus is on the use of local information expressed in the form of path constraints in the optimization of path expression queries. In particular, decidability and complexity results on the implication for path constraints are established.

Validating streaming XML documents

This paper investigates the on-line validation of streaming XML documents with respect to a DTD, under memory constraints. We first consider validation using constant memory, formalized by a finite-state automaton (FSA). We examine two flavors of the problem, depending on whether or not the XML document is assumed to be well-formed. The main results of the paper provide conditions on the DTDs under which validation of either flavor can be done using an FSA. For DTDs that cannot be validated by an FSA, we investigate two alternatives. The first relaxes the constant memory requirement by allowing a stack bounded in the depth of the XML document, while maintaining the deterministic, one-pass requirement. The second approach consists in refining the DTD to provide additional information that allows validation by an FSA.

A Web Odyssey: from Codd to XML

The Web presents the database area with vast opportunities and commensurate challenges. Databases and the Web are organically connected at many lev els. Web sites are increasingly pow ered b y databases.Collections of linked Web pages distributed across the Internet are themselves tempting targets for a database. The emergence of XML as the lingua franc a of the Web brings some m uchneeded order and will greatly facilitate the use of database techniques to manage Web information. This paper will discuss some of the developments related to the Web from the viewpoint of database theory. As we shall see, the Web scenario requires revisiting some of the basic assumptions of the area. T o be sure, database theory remains as valid as ev er in the classical setting, and the database industry will continue to represent a multi-billion dollar target of applicability for the foreseeable future. But the Web represents an opportunity of an entirely di erent scale. We are th us at an important juncture. Database theory could retain its classical focus and turn in w ards.Or, it could attempt to take heads-on the challenge of the Web and contribute to an important part of its formal foundations. T o do so, it will have to leave its familiar shores and reinvent itself. There are good signs that the journey has already begun. What makes theWeb scenario di erent from classical databases? In short, everything. A classical database is a coheren tly designed system. The system imposes rigid structure, and provides queries, updates, as well as transactions, concurrency, integrity, and recovery, in a con trolled environment. The Web escapes any suc h con trol.It is a freeevolving, ever-changing collection of data sources of various

Relational transducers for electronic commerce

Electronic commerce is emerging as one of the major Web-supported applications requiring database support. We introduce and study high-level declarative specifications of business models, using an approach in the spirit of active databases. More precisely, business models are specified as relational transducers that map sequences of input relations into sequences of output relations. The semantically meaningful trace of an input?output exchange is kept as a sequence of log relations. We consider problems motivated by electronic commerce applications, such as log validation, verifying temporal properties of transducers, and comparing two relational transducers. Positive results are obtained for a restricted class of relational transducers called Spocus transducers (for semi-positive outputs and cumulative state). We argue that despite the restrictions, these capture a wide range of practically significant business models.

Representing and querying XML with incomplete information

We study the representation and querying of XML with incomplete information. We consider a simple model for XML data and their DTDs, a very simple query language, and a representation system for incomplete information in the spirit of the representations systems developed by Imielinski and Lipski [1984] for relational databases. In the scenario we consider, the incomplete information about an XML document is continuously enriched by successive queries to the document. We show that our representation system can represent partial information about the source document acquired by successive queries, and that it can be used to intelligently answer new queries. We also consider the impact on complexity of enriching our representation system or query language with additional features. The results suggest that our approach achieves a practically appealing balance between expressiveness and tractability.