Merging schemas

There has been quite a bit of work on automatically creating mediated schemas that focused on the theoretical analysis of the semantics of merging schemas and the choices that need to be made in the process (Batini et al. 1986 Buneman et al. 1992 Hull 1984 Kalinichenko 1990 Miller et al. 1993 Pottinger and Bernstein 2002). The goal of these works was to make as many decisions automatically as possible, but where some ambiguity arises, refer to input from a designer. 

Abstract Merging schemas or other structured data occur in many different data models and applications, including merging ontologies, view integration, data integration, and computer supported collaborative work. This paper describes some of the key works in merging schemas and discusses some of the commonalities and differences. 

Merging and restructuring Now that the schemas are ready to be superimposed, how should they be combined Batini et al. [1986] offers the following qualitative criteria to decide on the correctness of the merged schema ... 

This kind of work shows the fundamental issues that have to be explored to merge schemas regardless of application or data model. 

Theorem 10 (Merge Equivalence). For all queries Q, the answers obtained by posing Q over a p-med-schema M = M Mi) with p-mappings pM, ..., pMi is equal to the answers obtained by posing Q over the consolidated mediated schema T with consolidated p-mapping pM. ... 

Schemas, ontologies and other related structures commonly need to be merged in a number of different applications. This happens for a number of reasons. For example ...

Convent [1986] concentrates primarily on what it means to have incompatible constraints. Informally, this means that if users are trying to integrate views, then for each user s view, it should be possible to access those instances from the global schema - note that this is very similar to the information capacity requirement laid out in Sect. 2.1 by Miller et al. [1993], Unfortunately, Convent [1986] shows that having incompatible constraints is undecidable even in this very basic case. Because of this early undecidability result, schema merging works typically do not consider instance-level constraints. 

Based on these requirements, the outcome of the merge is a set of relations, where each of the relations of the first three types of constraints (i.e., all but containment constraints) mean that the relations are combined, with all of their attributes present in the global schema. For containment constraints, the two relations are left separate, since there can be no way of determining what relational algebra operator to use. 

The combination phase consists of gathering all of the views, as well as determining the dependencies between schemas. The optimization phase concentrates on both minimizing the schema and reducing redundancy. They concentrate on the optimization phase, which is akin to the problem considered in semantic merge. 

There are a number of other schema merging works in the view integration domain around this period, including Shoval and Zohn [1991] and Navathe and Gadgil [1982], Generally, these works build on approximately the same foundation define what it means to merge and what it means for there to be a conflict. Most... 

This work is notable because it marks a departure of focus from the existing literature it concentrates on the fact that type information may conflict (rules 1 and 2) as well as the fact that some relationships that exist explicitly in the input schemas can be represented implicitly in the resulting schema (rules 3 and 5). This use of implicit relationships is featured prominently in later work, particularly in generic schema merging (Sect. 6). It is probably not a coincidence that this work, like the generic work, is not based in the relational model the more direct representation of relationships between schema elements allows the authors to deal with complex relationships without running into the undecidability result in Convent [1986],... 

These works represent the type of focus on schema merging present in the more recent view integration literature. After this point, more of the database research in schema merging came from data integration (Sect. 4) and generic schema management (Sect. 6). 

Generic Schema Merging 6.1 Buneman, Davidson, and Kosky... 

Fig. 8.3 Buneman et al. [1992] show that one conflict that occurs during the merging of schemas is that if there are two elements in the resulting merge that have different types (a), then a new type can be created which inherits from both original types (b)...

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