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.

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.