Global schemas are more valuable if they represent a clear view of the component databases.
For example, they should not be affected by implementation considerations, such as limits on the number of classes, tables, or attributes. Almost all heterogeneous database systems involve schema converters. Schema converters embody or are associated with data converters in that they define the way data structured according to one schema are converted to data structured according to another schema. Converting schemas and data is a non-trivial process especially when there is a substantial gap between data models. A heterogeneous database system can involve as many converters as the number of different pairs of databases in the system, where each converter would map between a pair of specific databases.