By Marcelo Arenas, Pablo Barcelo, Leonid Libkin, Filip Murlak, M. Tamer Ozsu
Info trade is the matter of discovering an example of a objective schema, given an example of a resource schema and a specification of the connection among the resource and the objective. this sort of aim example should still safely signify details from the resource example below the limitations imposed by means of the objective schema, and it's going to enable one to judge queries at the aim example in a fashion that's semantically in step with the resource facts. facts trade is an previous challenge that re-emerged as an lively learn subject lately, as a result of the elevated want for alternate of information in quite a few codecs, usually in e-business purposes. during this lecture, we provide an summary of the fundamental ideas of information trade in either relational and XML contexts. We supply examples of information trade difficulties, and we introduce the most projects that have to addressed. We then speak about relational facts alternate, targeting concerns equivalent to relational schema mappings, materializing aim circumstances (including canonical strategies and cores), question answering, and question rewriting. After that, we talk about metadata administration, i.e., dealing with schema mappings themselves. We pay specific consciousness to operations on schema mappings, comparable to composition and inverse. eventually, we describe either info alternate and metadata administration within the context of XML. We use mappings in keeping with remodeling tree styles, and we exhibit that they result in a bunch of latest difficulties that didn't come up within the relational case, yet they should be addressed for XML. those contain consistency matters for mappings and schemas, in addition to enforcing tighter regulations on mappings and queries to accomplish tractable question answering in information trade. desk of Contents: evaluation / Relational Mappings and information trade / Metadata administration / XML Mappings and information trade
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We can draw an interesting conclusion from this example: among all possible universal solutions, the canonical universal solution is not necessarily the smallest (as T ∗ is strictly contained in T ). Moreover, in the example, T ∗ is actually the smallest universal solution (up to isomorphism). The first natural question is whether there is always a unique smallest universal solution. As we will see later, this question has a positive answer. Further, some authors have argued that this smallest universal solution is the “best” universal solution since it is the most economical one in terms of size, and that this solution should be the preferred one at the moment of materializing a solution.
We conclude this section by saying a few words about this. More specifically, there exists an exponential-time algorithm that given two mappings M12 and M23 , each specified by an SO tgd, returns a mapping M13 specified by an SO tgd and equivalent to the composition of M12 and M23 . 6, can also be used to compute an SO tgd that defines the composition of two mappings given by st-tgds as every mapping specified by a finite set of st-tgds can be transformed into an equivalent mapping specified by an SO tgd.
Specifically, we concentrate on the following issues for each of these notions: the definition of its semantics, the study of the language needed to express it and the algorithmic issues associated to the problem of computing it. 2. 2 39 COMPOSITION OF SCHEMA MAPPINGS The composition operator has been identified as one of the fundamental operators for the development of a framework for managing schema mappings. The goal of this operator is to generate a mapping M13 that has the same effect as applying successively two given mappings M12 and M23 , provided that the target schema of M12 is the same as the source schema of M23 .