112,267 research outputs found
Covers of Query Results
We introduce succinct lossless representations of query results called covers. They are subsets of the query results that correspond to minimal edge covers in the hypergraphs of these results.
We first study covers whose structures are given by fractional hypertree decompositions of join queries.
For any decomposition of a query, we give asymptotically tight size bounds for the covers of the query result over that decomposition and show that such covers can be computed in worst-case optimal time up to a logarithmic factor in the database size. For acyclic join queries, we can compute covers compositionally using query plans with a new operator called cover-join. The tuples in the query result can be enumerated from any of its covers with linearithmic pre-computation time and constant delay.
We then generalize covers from joins to functional aggregate queries that express a host of computational problems such as aggregate-join queries, in-database optimization, matrix chain multiplication, and inference in probabilistic graphical models
Querying the Guarded Fragment
Evaluating a Boolean conjunctive query Q against a guarded first-order theory
F is equivalent to checking whether "F and not Q" is unsatisfiable. This
problem is relevant to the areas of database theory and description logic.
Since Q may not be guarded, well known results about the decidability,
complexity, and finite-model property of the guarded fragment do not obviously
carry over to conjunctive query answering over guarded theories, and had been
left open in general. By investigating finite guarded bisimilar covers of
hypergraphs and relational structures, and by substantially generalising
Rosati's finite chase, we prove for guarded theories F and (unions of)
conjunctive queries Q that (i) Q is true in each model of F iff Q is true in
each finite model of F and (ii) determining whether F implies Q is
2EXPTIME-complete. We further show the following results: (iii) the existence
of polynomial-size conformal covers of arbitrary hypergraphs; (iv) a new proof
of the finite model property of the clique-guarded fragment; (v) the small
model property of the guarded fragment with optimal bounds; (vi) a
polynomial-time solution to the canonisation problem modulo guarded
bisimulation, which yields (vii) a capturing result for guarded bisimulation
invariant PTIME.Comment: This is an improved and extended version of the paper of the same
title presented at LICS 201
Equality-friendly well-founded semantics and applications to description logics
We tackle the problem of deļ¬ning a well-founded semantics (WFS) for Datalog rules with existentially quantiļ¬ed variables in their heads and nega- tions in their bodies. In particular, we provide a WFS for the recent DatalogĀ± family of ontology languages, which covers several important description logics (DLs). To do so, we generalize DatalogĀ± by non-stratiļ¬ed nonmonotonic nega- tion in rule bodies, and we deļ¬ne a WFS for this generalization via guarded ļ¬xed point logic. We refer to this approach as equality-friendly WFS, since it has the advantage that it does not make the unique name assumption (UNA); this brings it close to OWL and its proļ¬les as well as typical DLs, which also do not make the UNA. We prove that for guarded DatalogĀ± with negation under the equality- friendly WFS, conjunctive query answering is decidable, and we provide precise complexity results for this problem. From these results, we obtain precise deļ¬- nitions of the standard WFS extensions of EL and of members of the DL-Lite family, as well as corresponding complexity results for query answering
From Causes for Database Queries to Repairs and Model-Based Diagnosis and Back
In this work we establish and investigate connections between causes for
query answers in databases, database repairs wrt. denial constraints, and
consistency-based diagnosis. The first two are relatively new research areas in
databases, and the third one is an established subject in knowledge
representation. We show how to obtain database repairs from causes, and the
other way around. Causality problems are formulated as diagnosis problems, and
the diagnoses provide causes and their responsibilities. The vast body of
research on database repairs can be applied to the newer problems of computing
actual causes for query answers and their responsibilities. These connections,
which are interesting per se, allow us, after a transition -inspired by
consistency-based diagnosis- to computational problems on hitting sets and
vertex covers in hypergraphs, to obtain several new algorithmic and complexity
results for database causality.Comment: To appear in Theory of Computing Systems. By invitation to special
issue with extended papers from ICDT 2015 (paper arXiv:1412.4311
Decomposing Complex Queries for Tip-of-the-tongue Retrieval
When re-finding items, users who forget or are uncertain about identifying
details often rely on creative strategies for expressing their information
needs -- complex queries that describe content elements (e.g., book characters
or events), information beyond the document text (e.g., descriptions of book
covers), or personal context (e.g., when they read a book). This retrieval
setting, called tip of the tongue (TOT), is especially challenging for models
heavily reliant on lexical and semantic overlap between query and document
text. In this work, we introduce a simple yet effective framework for handling
such complex queries by decomposing the query into individual clues, routing
those as sub-queries to specialized retrievers, and ensembling the results.
This approach allows us to take advantage of off-the-shelf retrievers (e.g.,
CLIP for retrieving images of book covers) or incorporate retriever-specific
logic (e.g., date constraints). We show that our framework incorportating query
decompositions into retrievers can improve gold book recall up to 7% relative
again for Recall@5 on a new collection of 14,441 real-world query-book pairs
from an online community for resolving TOT inquiries
Foundations of Rule-Based Query Answering
This survey article introduces into the essential concepts and methods underlying rule-based query languages. It covers four complementary areas: declarative semantics based on adaptations of mathematical logic, operational semantics, complexity and expressive power, and optimisation of query evaluation.
The treatment of these areas is foundation-oriented, the foundations having resulted from over four decades of research in the logic programming and database communities on combinations of query languages and rules. These results have later formed the basis for conceiving, improving, and implementing several Web and Semantic Web technologies, in particular query languages such as XQuery or SPARQL for querying relational, XML, and RDF data, and rule languages like the āRule Interchange Framework (RIF)ā currently being developed in a working group of the W3C.
Coverage of the article is deliberately limited to declarative languages in a classical setting: issues such as query answering in F-Logic or in description logics, or the relationship of query answering to reactive rules and events, are not addressed
Characterization and exploitation of community structure in cover song networks
The use of community detection algorithms is explored within the framework of
cover song identification, i.e. the automatic detection of different audio
renditions of the same underlying musical piece. Until now, this task has been
posed as a typical query-by-example task, where one submits a query song and
the system retrieves a list of possible matches ranked by their similarity to
the query. In this work, we propose a new approach which uses song communities
to provide more relevant answers to a given query. Starting from the output of
a state-of-the-art system, songs are embedded in a complex weighted network
whose links represent similarity (related musical content). Communities inside
the network are then recognized as groups of covers and this information is
used to enhance the results of the system. In particular, we show that this
approach increases both the coherence and the accuracy of the system.
Furthermore, we provide insight into the internal organization of individual
cover song communities, showing that there is a tendency for the original song
to be central within the community. We postulate that the methods and results
presented here could be relevant to other query-by-example tasks
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