Leonid Libkin

Incremental maintenance of views with duplicates

We study the problem of efficient maintenance of materialized views that may contain duplicates. This problem is particularly important when queries against such views involve aggregate functions, which need duplicates to produce correct results. Unlike most work on the view maintenance problem that is based on an algorithmic approach, our approach is algebraic and based on equational reasoning. This approach has a number of advantages: it is robust and easily extendible to new language constructs, it produces output that can be used by query optimizers, and it simplifies correctness proofs.We use a natural extension of the relational algebra operations to bags (multisets) as our basic language. We present an algorithm that propagates changes from base relations to materialized views. This algorithm is based on reasoning about equivalence of bag-valued expressions. We prove that it is correct and preserves a certain notion of minimality that ensures that no unnecessary tuples are computed. Although it is generally only a heuristic that computing changes to the view rather than recomputing the view from scratch is more efficient, we prove results saying that under normal circumstances one should expect, the change propagation algorithm to be significantly faster and more space efficient than complete recomputing of the view. We also show that our approach interacts nicely with aggregate functions, allowing their correct evaluation on views that change.

Reasoning about pattern-based XML queries

We survey results about static analysis of pattern-based queries over XML documents. These queries are analogs of conjunctive queries, their unions and Boolean combinations, in which tree patterns play the role of atomic formulae. As in the relational case, they can be viewed as both queries and incomplete documents, and thus static analysis problems can also be viewed as finding certain answers of queries over such documents. We look at satisfiability of patterns under schemas, containment of queries for various features of XML used in queries, finding certain answers, and applications of pattern-based queries in reasoning about schema mappings for data exchange.

XML data exchange: consistency and query answering

Data exchange is the problem of finding an instance of a target schema, given an instance of a source schema and a specification of the relationship between the source and the target. Theoretical foundations of data exchange have recently been investigated for relational data.In this paper, we start looking into the basic properties of XML data exchange, that is, restructuring of XML documents that conform to a source DTD under a target DTD, and answering queries written over the target schema. We define XML data exchange settings in which source-to-target dependencies refer to the hierarchical structure of the data. Combining DTDs and dependencies makes some XML data exchange settings inconsistent. We investigate the consistency problem and determine its exact complexity.We then move to query answering, and prove a dichotomy theorem that classifies data exchange settings into those over which query answering is tractable, and those over which it is coNP-complete, depending on classes of regular expressions used in DTDs. Furthermore, for all tractable cases we give polynomial-time algorithms that compute target XML documents over which queries can be answered.

A normal form for XML documents

This paper takes a first step towards the design and normalization theory for XML documents. We show that, like relational databases, XML documents may contain redundant information, and may be prone to update anomalies. Furthermore, such problems are caused by certain functional dependencies among paths in the document. Our goal is to find a way of converting an arbitrary DTD into a well-designed one, that avoids these problems. We first introduce the concept of a functional dependency for XML, and define its semantics via a relational representation of XML. We then define an XML normal form, XNF, that avoids update anomalies and redundancies. We study its properties and show that it generalizes BCNF and a normal form for nested relations when those are appropriately coded as XML documents. Finally, we present a lossless algorithm for converting any DTD into one in XNF.

A query language for multidimensional arrays: design, implementation, and optimization techniques

While much recent research has focussed on extending databases beyond the traditional relational model, relatively little has been done to develop database tools for querying data organized in (multidimensional) arrays. The scientific computing community has made little use of available database technology. Instead, multidimensional scientific data is typically stored in local files conforming to various data exchange formats and queried via specialized access libraries tied in to general purpose programming languages.To allow such data to be queried using known database techniques, we design and implement a query language for multidimensional arrays. Our main design decision is to treat arrays as functions from index sets to values rather than as collection types. This leads to clean syntax and semantics as well as simple but powerful optimization rules.We present a calculus for arrays that extends standard calculi for complex objects. We derive a higher-level comprehension style query language based on this calculus and describe its implementation, including a data driver for the NetCDF data exchange format. Next, we explore some optimization rules obtained from the equational laws of our core calculus. Finally, we study the expressiveness of our calculus and prove that it essentially corresponds to adding ranking to a query language for complex objects.

Data exchange and incomplete information

Data exchange is the problem of finding an instance of a target schema, given an instance of a source schema and a specification of the relationship between the source and the target, and answering queries over target instances in a way that is semantically consistent with the information in the source. Theoretical foundations of data exchange have been actively explored recently. It was also noticed that the standard certain answers semantics may behave in very odd ways.In this paper I explain that this behavior is due to the fact that the presence of incomplete information in target instances has been ignored; in particular, proper query evaluation techniques for databases with nulls have not been used, and the distinction between closed and open world semantics has not been made. I present a concept of target solutions based on the closed world assumption, and show that the space of all solutions has two extreme points: the canonical universal solution and the core, well studied in data exchange. I show how to define semantics of query answering taking into account incomplete information, and show that the well-known anomalies go away with the new semantics. The paper also contains results on the complexity of query answering, upper approximations to queries (maybe-answers), and various extensions.

On XML integrity constraints in the presence of DTDs

The paper investigates XML document specifications with DTDs and integrity constraints, such as keys and foreign keys. We study the consistency problem of checking whether a given specification is meaningful: that is, whether there exists an XML document that both conforms to the DTD and satisfies the constraints. We show that DTDs interact with constraints in a highly intricate way and as a result, the consistency problem in general is undecidable. When it comes to unary keys and foreign keys, the consistency problem is shown to be NP-complete. This is done by coding DTDs and integrity constraints with linear constraints on the integers. We consider the variations of the problem (by both restricting and enlarging the class of constraints), and identify a number of tractable cases, as well as a number of additional NP-complete ones. By incorporating negations of constraints, we establish complexity bounds on the implication problem, which is shown to be coNP-complete for unary keys and foreign keys.

Database Theory - ICDT 2005

Title Type database theory icdt 2005 10th international conference edinburgh uk january 5 7 2005 proceed PDF user modeling 2005 10th international conference um 2005 edinburgh scotland uk july 24 29 2005 PDF database theory icdt 95 5th international conference prague czech republic january 11 13 19 PDF algorithmic learning theory 16th international conference alt 2005 singapore october 8 11 2005 PDF trustworthy global computing international symposium tgc 2005 edinburgh uk april 7 9 2005 revi PDF database programming languages 10th international symposium dbpl 2005 trondheim norway august 28 PDF implementation and application of automata 10th international conference ciaa 2005 sophia antipoli PDF trust management third international conference itrust 2005 paris france may 23 26 2005 procee PDF image and video retrieval 4th international conference civr 2005 singapore july 20 22 2005 proc PDF logic programming 21st international conference iclp 2005 sitges spain october 2 5 2005 procee PDF advances in cryptology asiacrypt 2005 11th international conference on the theory and application PDF spatial information theory international conference cosit 2005 ellicottville ny usa september 1 PDF cryptographic hardware and embedded systems ches 2005 7th international workshop edinburgh uk a PDF formal approaches to software testing 5th international workshop fates 2005 edinburgh uk july 11 PDF international institutional reform 2005 hague joint conference on issues of international law PDF the semantic web iswc 2005 4th international semantic web conference iswc 2005 galway ireland PDF topics in cryptology ct rsa 2005 the cryptographers track at the rsa conference 2005 san francis PDF ki 2005 advances in artificial intelligence 28th annual german conference on ai ki 2005 koblenz PDF theoretical computer science 9th italian conference ictcs 2005 siena italy october 12 14 2005 PDF icame 2005 proceedings of the 28th international conference on the applications of the mssbauer eff PDF algorithms for approximation proceedings of the 5th international conference chester july 2005 PDF proceedings of the international conference on natural language processing icon 2005 PDF wiredwireless internet communications third international conference wwic 2005 xanthi greece ma PDF unconventional computation 4th international conference uc 2005 sevilla spain october 3 7 proce PDF logic programming and nonmonotonic reasoning 8th international conference lpnmr 2005 diamante ita PDF computational intelligence and security international conference cis 2005 xian china december 1 PDF fundamental approaches to software engineering 8th international conference fase 2005 held as par PDF intelligent virtual agents 5th international working conference iva 2005 kos greece september 12 PDF entertainment computing icec 2005 4th international conference sanda japan september 19 21 200 PDF distributed computing and internet technology second international conference icdcit 2005 bhubanes PDF mobile ad hoc and sensor networks first international conference msn 2005 wuhan china december 1 PDF information security practice and experience first international conference ispec 2005 singapore PDF high performance computing and communications first international conference hpcc 2005 sorrento i PDF advances in web age information management 6th international conference waim 2005 hangzhou china PDF advances in machine learning and cybernetics 4th international conference icmlc 2005 guangzhou ch PDF advances in computer systems architecture 10th asia pacific conference acsac 2005 singapore octob PDF developments in language theory 10th international conference dlt 2006 santa barbara ca usa jun PDF advances in cryptology asiacrypt 2004 10th international conference on the theory and application PDF logical aspects of computational linguistics 5th international conference lacl 2005 bordeaux fran PDF

XML data exchange: Consistency and query answering

Data exchange is the problem of finding an instance of a target schema, given an instance of a source schema and a specification of the relationship between the source and the target. Theoretical foundations of data exchange have recently been investigated for relational data. In this article, we start looking into the basic properties of XML data exchange, that is, restructuring of XML documents that conform to a source DTD under a target DTD, and answering queries written over the target schema. We define XML data exchange settings in which source-to-target dependencies refer to the hierarchical structure of the data. Combining DTDs and dependencies makes some XML data exchange settings inconsistent. We investigate the consistency problem and determine its exact complexity. We then move to query answering, and prove a dichotomy theorem that classifies data exchange settings into those over which query answering is tractable, and those over which it is coNP-complete, depending on classes of regular expressions used in DTDs. Furthermore, for all tractable cases we give polynomial-time algorithms that compute target XML documents over which queries can be answered.

Expressive Languages for Path Queries over Graph-Structured Data

For many problems arising in the setting of graph querying (such as finding semantic associations in RDF graphs, exact and approximate pattern matching, sequence alignment, etc.), the power of standard languages such as the widely studied conjunctive regular path queries (CRPQs) is insufficient in at least two ways. First, they cannot output paths and second, more crucially, they cannot express relationships among paths. We thus propose a class of extended CRPQs, called ECRPQs, which add regular relations on tuples of paths, and allow path variables in the heads of queries. We provide several examples of their usefulness in querying graph structured data, and study their properties. We analyze query evaluation and representation of tuples of paths in the output by means of automata. We present a detailed analysis of data and combined complexity of queries, and consider restrictions that lower the complexity of ECRPQs to that of relational conjunctive queries. We study the containment problem, and look at further extensions with first-order features, and with nonregular relations that add arithmetic constraints on the lengths of paths and numbers of occurrences of labels.