Transdata database12/8/2022 ![]() ![]() Abiteboul, S., Hull, R., and Vianu, V.Moreover, we show that maximum extended recoveries can be used to capture in a quantitative way, the amount of information loss embodied in a schema mapping specified by source-to-target tuple-generating dependencies. We give results concerning the existence of extended inverses and of maximum extended recoveries, and results concerning their applications to reverse data exchange and query answering. To this effect, we introduce the notions of extended identity schema mapping, extended inverse, and maximum extended recovery, by making systematic use of the homomorphism relation on instances. The development of this new framework requires a careful reformulation of all the important notions, including the notions of the identity schema mapping, inverse, and maximum recovery. We develop a new framework for reverse data exchange that supports source instances that may contain nulls, and we thereby overcome the semantic mismatch between source and target instances of the previous formalizations. This restriction on source instances is crucial for obtaining some of the main technical results about these three notions, but, at the same time, limits their usefulness, since reverse data exchange naturally leads to source instances that may contain both constants and labeled nulls. The study of these notions has been carried out in the context in which source instances are restricted to consist entirely of constants, while target instances may contain both constants and labeled nulls. In recent years, three different formalizations of this concept have been introduced and studied, namely the notions of an inverse of a schema mapping, a quasi-inverse of a schema mapping, and a maximum recovery of a schema mapping. ![]() An inverse of a schema mapping M is intended to undo what M does, thus providing a way to perform reverse data exchange. ![]()
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