Deep Separability of Ontological Constraints
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by
Andrea Calì, Marco Console, Riccardo Frosini
2013
Abstract
When data schemata are enriched with expressive constraints that aim at
representing the domain of interest, in order to answer queries one needs to
consider the logical theory consisting of both the data and the constraints.
Query answering in such a context is called ontological query answering.
Commonly adopted database constraints in this field are tuple-generating
dependencies (TGDs) and equality-generating dependencies (EGDs). It is well
known that their interaction leads to intractability or undecidability of query
answering even in the case of simple subclasses. Several conditions have been
found to guarantee separability, that is lack of interaction, between TGDs and
EGDs. Separability makes EGDs (mostly) irrelevant for query answering and
therefore often guarantees tractability, as long as the theory is satisfiable.
In this paper we review the two notions of separability found in the
literature, as well as several syntactic conditions that are sufficient to
prove them. We then shed light on the issue of satisfiability checking, showing
that under a sufficient condition called deep separability it can be done by
considering the TGDs only.
We show that, fortunately, in the case of TGDs and EGDs, separability implies
deep separability. This result generalizes several analogous ones, proved ad
hoc for particular classes of constraints. Applications include the class of
sticky TGDs and EGDs, for which we provide a syntactic separability condition
which extends the analogous one for linear TGDs; preliminary experiments show
the feasibility of query answering in this case.
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