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24
Locally consistent transformations and query answering in data exchange
 PODS
, 2004
"... Data exchange is the problem of taking data structured under a source schema and creating an instance of a target schema. Given a source instance, there may be many solutions – target instances that satisfy the constraints of the data exchange problem. Previous work has identified two classes of des ..."
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Cited by 50 (19 self)
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Data exchange is the problem of taking data structured under a source schema and creating an instance of a target schema. Given a source instance, there may be many solutions – target instances that satisfy the constraints of the data exchange problem. Previous work has identified two classes of desirable solutions: canonical universal solutions, and their cores. Query answering in data exchange amounts to rewriting a query over the target schema to another query that, over a materialized target instance, gives the result that is semantically consistent with the source. A basic question is then whether there exists a transformation sending a source instance into a solution over which target queries can be answered. We show that the answer is negative for many data exchange transformations that have structural properties similar to canonical universal solutions and cores. Namely, we prove that many such transformations preserve the local structure of the data. Using this notion, we further show that every target query rewritable over such a transformation cannot distinguish tuples whose neighborhoods in the source are similar. This gives us a first tool that helps check whether a query is rewritable. We also show that these results are robust: they hold for an extension of relational calculus with grouping and aggregates, and for two different semantics of query answering.
Configuration knowledge representations for Semantic Web applications
, 2003
"... Today’s economy exhibits a growing trend toward highly specialized solution providers cooperatively offering configurable products and services to their customers. This paradigm shift requires the extension of current standalone configuration technology with capabilities of knowledge sharing and dis ..."
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Cited by 22 (11 self)
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Today’s economy exhibits a growing trend toward highly specialized solution providers cooperatively offering configurable products and services to their customers. This paradigm shift requires the extension of current standalone configuration technology with capabilities of knowledge sharing and distributed problem solving. In this context a standardized configuration knowledge representation language with formal semantics is needed in order to support knowledge interchange between different configuration environments. Languages such as Ontology Inference Layer (OIL) and DARPA Agent Markup Language (DAML+OIL) are based on such formal semantics (description logic) and are very popular for knowledge representation in the Semantic Web. In this paper we analyze the applicability of those languages with respect to configuration knowledge representation and discuss additional demands on expressivity. For joint configuration problem solving it is necessary to agree on a common problem definition. Therefore, we give a description logic based definition of a configuration problem and show its equivalence with existing consistencybased definitions, thus joining the two major streams in knowledgebased configuration (description logics and predicate
Containment of aggregate queries
 In Proc. of ICDT
, 2003
"... Abstract. The problem of deciding containment of aggregate queries is investigated. Containment is reduced to equivalence for queries with expandable aggregation functions. Many common aggregation functions, such as max, cntd (count distinct), count, sum, avg, median and stdev (standard deviation) a ..."
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Cited by 21 (3 self)
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Abstract. The problem of deciding containment of aggregate queries is investigated. Containment is reduced to equivalence for queries with expandable aggregation functions. Many common aggregation functions, such as max, cntd (count distinct), count, sum, avg, median and stdev (standard deviation) are shown to be expandable. It is shown that even in the presence of integrity constraints, containment can be reduced to equivalence. For conjunctive count and sumqueries, simpler characterizations for containment are given, that do not require checking equivalence. These results are built upon in order to solve the problem of finding maximallycontained sets of rewritings for conjunctive countqueries. 1
Expressive Power of SQL
, 2001
"... It is a folk result in database theory that SQL cannot express recursive queries such as reachability; in fact, a new construct was added to SQL3 to overcome this limitation. However, the evidence for this claim is usually given in the form of a reference to a proof that relational algebra cannot ex ..."
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Cited by 16 (0 self)
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It is a folk result in database theory that SQL cannot express recursive queries such as reachability; in fact, a new construct was added to SQL3 to overcome this limitation. However, the evidence for this claim is usually given in the form of a reference to a proof that relational algebra cannot express such queries. SQL, on the other hand, in all its implementations has three features that fundamentally distinguish it from relational algebra: namely, grouping, arithmetic operations, and aggregation.
Equivalences among aggregate queries with negation
 In Proc. 20th Symposium on Principles of Database Systems
, 2001
"... Query equivalence is investigated for disjunctive aggregate queries with negated subgoals, constants and comparisons. A full characterization of equivalence is given for the aggregation functions count, max, sum, prod, top2 and parity. A related problem is that of determining, for a given natural nu ..."
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Cited by 12 (3 self)
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Query equivalence is investigated for disjunctive aggregate queries with negated subgoals, constants and comparisons. A full characterization of equivalence is given for the aggregation functions count, max, sum, prod, top2 and parity. A related problem is that of determining, for a given natural number N, whether two given queries are equivalent over all databases with at most N constants. We call this problem bounded equivalence. A complete characterization of decidability of bounded equivalence is given. In particular, it is shown that this problem is decidable for all the above aggregation functions as well as for cntd (count distinct) and avg. For quasilinear queries (i.e., queries where predicates that occur positively are not repeated) it is shown that equivalence can be decided in polynomial time for the aggregation functions count, max, sum, parity, prod, top2 and avg. A similar result holds for cntd provided that a few additional conditions hold. The results are couched in terms of abstract characteristics of aggregation functions, and new proof techniques are used. Finally, the results above also imply that equivalence, under bagset semantics, is decidable for nonaggregate queries with negation. 1
Logics with counting and local properties
 ACM Trans. on Computational Logic
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Incremental recomputation in local languages.
 Inf. Comput.
, 2003
"... Abstract We study the problem of maintaining recursively defined views, such as the transitive closure of a relation, in traditional relational languages that do not have recursion mechanisms. The main results of this paper are negative ones: we show that a certain property of query languages impli ..."
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Abstract We study the problem of maintaining recursively defined views, such as the transitive closure of a relation, in traditional relational languages that do not have recursion mechanisms. The main results of this paper are negative ones: we show that a certain property of query languages implies impossibility of such incremental maintenance. The property we use is locality of queries, which is known to hold for relational calculus and various extensions, including those with grouping and aggregate constructs (essentially, plain SQL).
An Existential Locality Theorem
"... We prove an existential version of Gaifman's locality theorem and show how it can be applied algorithmically to evaluate existential firstorder sentences in finite structures. ..."
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Cited by 6 (2 self)
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We prove an existential version of Gaifman's locality theorem and show how it can be applied algorithmically to evaluate existential firstorder sentences in finite structures.
Lower Bounds for Invariant Queries in Logics with Counting
 TCS
, 2002
"... We study the expressive power of counting logics in the presence of auxiliary relations such as orders and preorders. The simplest such logic, firstorder with counting, captures the complexity class TC 0 over ordered structures. We also consider firstorder logic with arbitrary unary quantifie ..."
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We study the expressive power of counting logics in the presence of auxiliary relations such as orders and preorders. The simplest such logic, firstorder with counting, captures the complexity class TC 0 over ordered structures. We also consider firstorder logic with arbitrary unary quantifiers, and infinitary extensions. We start by giving a simple direct proof that firstorder with counting, in the presence of preorders that are almosteverywhere linear orders, cannot express the transitive closure of a binary relation. The proof is based on locality of formulae. We then show that the technique cannot be extended to linear orders, and that the result does not say anything about the power of invariant queries in firstorder with counting, in the presence of those preorders, vs. the class TC 0 . In the second part of the paper we then prove a separation result showing that for all the counting logics above, a linear order is more powerful than a preorder that is a linea...