Database Reference
In-Depth Information
17
What is metadata management?
So far we have concentrated on handling data in data exchange, i.e., transforming source
databases into target ones, and answering queries over them. We now look at manipulating
information about schemas and schema mappings, known as metadata , i.e., we deal with
metadata management. In this short chapter we outline the key problems that need to be
addressed in the context of metadata management. These are divided into two groups of
problems. The first concerns reasoning about mappings, and the second group of problems
is about manipulating mappings, i.e., building new mappings from existing ones.
17.1 Reasoning about schema mappings
As we have seen, mappings are logical specifications of the relationship between schemas,
both in the relational and XML scenarios. In particular, we have seen many different logical
languages that are used to specify mappings. Thus, a first natural problem that one would
like to study in the context of metadata management is to characterize the properties that
a mapping satisfies depending on the logical formulae that are used to define it. More
precisely, one would like, in the first place, to understand whether the logical formulae
used to specify a mapping are excessively restrictive in the sense that no source instance
admits a solution, or at least restrictive in the sense that some source instances do not
admit solutions. Note that this is different from the problem of checking for the existence
of solutions, studied in Chapter 5 . In that problem, the input consists of both a mapping
and an instance, and we want to see if the mapping can be applied to a particular instance.
Now we are talking about just metadata management problems. That is, the input consists
of a mapping alone, and we want to know, before seeing any data, whether this mapping
actually makes sense. The latter could mean that it is either applicable to some instances,
which we shall term the consistency problem , or it is applicable to all instances, which we
shall term the absolute consistency problem. These consistency problems are studied in
Chapter 18 .
The second type of problems that we look at are of the following nature: we would like
to understand what one can lose or gain by switching from one language to another. In par-
ticular, we want to understand what happens to the fundamental tasks such as constructing
solutions or answering target queries when we switch between different logical languages
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