Alin Deutsch

Storing semistructured data with STORED

Systems for managing and querying semistructured-data sources often store data in proprietary object repositories or in a tagged-text format. We describe a technique that can use relational database management systems to store and manage semistructured data. Our technique relies on a mapping between the semistructured data model and the relational data model, expressed in a query language called STORED. When a semistructured data instance is given, a STORED mapping can be generated automatically using data-mining techniques. We are interested in applying STORED to XML data, which is an instance of semistructured data. We show how a document-type-descriptor (DTD), when present, can be exploited to further improve performance.

The chase revisited

We revisit the standard chase procedure, studying its properties and applicability to classical database problems. We settle (in the negative) the open problem of decidability of termination of the standard chase, and we provide sufficient termination conditions which are strictly less over-conservative than the best previously known. We investigate the adequacy of the standard chase for checking query containment under constraints, constraint implication and computing certain answers in data exchange, gaining a deeper understanding by separating the algorithm from its result. We identify the properties of the chase result that are essential to the above applications, and we introduce the more general notion of F-universal model set, which supports query and constraint languages that are closed under a class F of mappings. By choosing F appropriately, we extend prior results to existential first-order queries and ∀∃-firstorder constraints. We show that the standard chase is incomplete for finding universal model sets, and we introduce the extended core chase which is complete, i.e. finds an F-universal model set when it exists. A key advantage of the new chase is that the same algorithm can be applied for all mapping classes F of interest, simply by modifying the set of constraints given as input. Even when restricted to the typical input in prior work, the new chase supports certain answer computation and containment/implication tests in strictly more cases than the incomplete standard chase.

Reformulation of XML Queries and Constraints

We state and solve the query reformulation problem for XML publishing in a general setting that allows mixed (XML and relational) storage for the proprietary data and exploits redundancies (materialized views, indexes and caches) to enhance performance. The correspondence between published and proprietary schemas is specified by views in both directions, and the same algorithm performs rewriting-with-views, composition-with-views, or the combined effect of both, unifying the Global-As-View and Local-As-View approaches to data integration. We prove a completeness theorem which guarantees that under certain conditions, our algorithm will find a minimal reformulation if one exists. Moreover, we identify conditions when this algorithm achieves optimal complexity bounds. We solve the reformulation problem for constraints by exploiting a reduction to the problem of query reformulation.

MARS: a system for publishing XML from mixed and redundant storage

We present a system for publishing as XML data from mixed (relational+XML) proprietary storage, while supporting redundancy in storage for tuning purposes. The correspondence between public and proprietary schemas is given by a combination of LAV-and GAV-style views expressed in XQuery. XML and relational integrity constraints are also taken into consideration. Starting with client XQueries formulated against the public schema the system achieves the combined effect of rewriting-with-views, composition-with-views and query minimization under integrity constraints to obtain optimal reformulations against the proprietary schema. The paper focuses on the engineering and the experimental evaluation of the MARS system.

Query reformulation with constraints

Let Σ<inf>1</inf>, Σ<inf>2</inf> be two schemas, which may overlap, <i>C</i> be a set of constraints on the joint schema Σ<inf>1</inf> ∪ Σ<inf>2</inf>, and <i>q</i><inf>1</inf> be a Σ<inf>1</inf>-query. An <b>(equivalent) reformulation</b> of <i>q</i><inf>1</inf> in the presence of <i>C</i> is a Σ<inf>2</inf>-query, <i>q</i><inf>2</inf>, such that <i>q</i><inf>2</inf> gives the same answers as <i>q</i><inf>1</inf> on any Σ<inf>1</inf> ∪ Σ<inf>2</inf>-database instance that satisfies <i>C</i>. In general, there may exist multiple such reformulations and choosing among them may require, for example, a cost model.

Specification and verification of data-driven Web applications

We study data-driven Web applications provided by Web sites interacting with users or applications. The Web site can access an underlying database, as well as state information updated as the interaction progresses, and receives user input. The structure and contents of Web pages, as well as the actions to be taken, are determined dynamically by querying the underlying database as well as the state and inputs. The properties to be verified concern the sequences of events (inputs, states, and actions) resulting from the interaction, and are expressed in linear or branching-time temporal logics. The results establish under what conditions automatic verification of such properties is possible and provide the complexity of verification. This brings into play a mix of techniques from logic and model checking.

ASTERIX: towards a scalable, semistructured data platform for evolving-world models

ASTERIX is a new data-intensive storage and computing platform project spanning UC Irvine, UC Riverside, and UC San Diego. In this paper we provide an overview of the ASTERIX project, starting with its main goal—the storage and analysis of data pertaining to evolving-world models. We describe the requirements and associated challenges, and explain how the project is addressing them. We provide a technical overview of ASTERIX, covering its architecture, its user model for data and queries, and its approach to scalable query processing and data management. ASTERIX utilizes a new scalable runtime computational platform called Hyracks that is also discussed at an overview level; we have recently made Hyracks available in open source for use by other interested parties. We also relate our work on ASTERIX to the current state of the art and describe the research challenges that we are currently tackling as well as those that lie ahead.

Specification and verification of data-driven web services

We study data-driven Web services provided by Web sites interacting with users or applications. The Web site can access an underlying database, as well as state information updated as the interaction progresses, and receives user input. The structure and contents of Web pages, as well as the actions to be taken, are determined dynamically by querying the underlying database as well as the state and inputs. The properties to be verified concern the sequences of events (inputs, states, and actions) resulting from the interaction, and are expressed in linear or branching-time temporal logics. The results establish under what conditions automatic verification of such properties is possible and provide the complexity of verification. This brings into play a mix of techniques from logic and automatic verification.

Rewriting queries using views with access patterns under integrity constraints

We study the problem of rewriting queries using views in the presence of access patterns, integrity constraints, disjunction, and negation. We provide asymptotically optimal algorithms for finding minimal containing and maximal contained rewritings and for deciding whether an exact rewriting exists. We show that rewriting queries using views in this case reduces (a) to rewriting queries with access patterns and constraints without views and also (b) to rewriting queries using views under constraints without access patterns. We show how to solve (a) directly and how to reduce (b) to rewriting queries under constraints only (semantic optimization). These reductions provide two separate routes to a unified solution for all three problems, based on an extension of the relational chase theory to queries and constraints with disjunction and negation. We also handle equality and arithmetic comparisons.

Verification of communicating data-driven web services

We study the verification of compositions of Web Service peers which interact asynchronously by exchanging messages. Each peer has access to a local database and reacts to user input and incoming messages by performing various actions and sending messages. The reaction is described by queries over the database, internal state, user input and received messages. We consider two formalisms for specification of correctness properties of compositions, namely Linear Temporal First-Order Logic and Conversation Protocols. For both formalisms, we map the boundaries of verification decidability, showing that they include expressive classes of compositions and properties. We also address modular verification, in which the correctness of a composition is predicated on the properties of its environment.