On the representation and querying of sets of possible worlds

Abiteboul, Serge; Kanellakis, Paris C.; Grahne, Gösta

doi:10.1016/0304-3975(51)90007-2

Cited by 184 publications

(140 citation statements)

References 12 publications

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“…Since we know how to construct canonical solutions and cores [10,11,24,13], the problem of answering queries in data exchange is thus reduced to the classical and well studied problem of answering queries in databases with incomplete information [2,3,17].…”

Section: Query Answering: Semanticsmentioning

confidence: 99%

“…Semantics data complexity certain 2 coNP-complete certain 3 coNP-complete maybe 2 NP-complete maybe 3 NP-complete The data complexity of FO queries for the semantics certain 2 , certain 3 , maybe 2 , and maybe 3 is as shown in Figure 2.…”

Section: Query Answering: Complexitymentioning

confidence: 99%

“…Of course it has long been known that the complexity of computing certain (maybe) answers for FO queries is coNPcomplete (NP-complete, respectively) [1,3], in the size of a table T . However, here we measure the complexity of answering queries on CanSol(S) or Core(S), in terms of the size of S. Thus, in the proof of Theorem 5.1, we provide hardness examples that, unlike those in [1,3], arise as CanSol(S) or Core(S) for some fixed data exchange setting.…”

Section: Query Answering: Complexitymentioning

confidence: 99%

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Data exchange and incomplete information

Libkin

2006

Proceedings of the Twenty-Fifth ACM SIGMOD-SIGACT-SIGART Symposium on Principles of Database Systems

106

157

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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.

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Section: Query Answering: Semanticsmentioning

confidence: 99%

Section: Query Answering: Complexitymentioning

confidence: 99%

Section: Query Answering: Complexitymentioning

confidence: 99%

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Data exchange and incomplete information

Libkin

2006

Proceedings of the Twenty-Fifth ACM SIGMOD-SIGACT-SIGART Symposium on Principles of Database Systems

106

157

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“…To this end, we define approximations of possible answers and two matching-based semijoin operators. There already exists a notion of maybeanswers [2,31] -answers that appear in Q v(D) for at least one valuation v -but those can be infinite, and include arbitrary elements outside of adom(D). What we need instead is a compact representation.…”

Section: An Implementation-friendly Translationmentioning

confidence: 99%

“…The reason is that SQL's designers had first and foremost efficient evaluation in mind, but correctness and efficiency do not always get along. Computing certain answers is CONP-hard for most reasonable semantics, if we deal with relational calculus/algebra queries [2]. On the other hand, SQL evaluation is very efficient; it is in AC 0 (a small parallel complexity class) for the same class of queries, and so it provably cannot compute certain answers.…”

Section: Introductionmentioning

confidence: 99%

Correctness of SQL Queries on Databases with Nulls

Guagliardo

Libkin

2017

SIGMOD Rec.

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Multiple issues with SQL's handling of nulls have been well documented. Having efficiency as its main goal, SQL disregards the standard notion of correctness on incomplete databases -certain answers -due to its high complexity. As a result, the evaluation of SQL queries on databases with nulls may produce answers that are just plain wrong. However, SQL evaluation can be modified, at least for relational algebra queries, to approximate certain answers, i.e., return only correct answers. We examine recently proposed approximation schemes for certain answers and analyze their complexity, both theoretical bounds and real-life behavior.

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About yes/no queries against possibilistic databases

Bosc

Pivert

2007

Int. J. Intell. Syst.

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This article is concerned with the handling of imprecise information in databases. The need for dealing with imprecise data is more and more acknowledged in order to cope with real data, even if commercial systems are most of the time unable to manage them. Here, the possibilistic setting is taken into consideration because it is less demanding than the probabilistic one. Then, any imprecise piece of information is modeled as a possibility distribution intended for constraining the more or less acceptable values. Such a possibilistic database has a natural interpretation in terms of a set of regular databases, which provides the basic gateway to interpret queries. However, if this approach is sound, it is not realistic, and it is necessary to consider restricted queries for which a calculus grounded on the possibilistic database, that is, where the operators work directly on possibilistic relations, is feasible. Extended yes/no queries are dealt with here, where their general form is: "to what extent is it possible and certain that tuple t~given! belongs to the answer to Q," where Q is an algebraic relational query. A strategy for processing such queries efficiently is proposed under some assumptions as to the operators appearing in Q.

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On the representation and querying of sets of possible worlds

Cited by 184 publications

References 12 publications

Data exchange and incomplete information

Data exchange and incomplete information

Correctness of SQL Queries on Databases with Nulls

About yes/no queries against possibilistic databases

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