Some MBDs, such as GenBank, do not provide such facilities. Translating queries between different query languages can be very complex if the query languages are based on different data models. Query processing can involve a global query optimization stage that determines a strategy for accessing the component databases and combining the query results. Note that this global query optimization complements the query optimizers of the component databases if such optimizers are available and must take into account the peculiarities of these optimizers. The optimization techniques also depend on the type of query interface supported by the system.
Retrieval queries are easier to support in a heterogeneous database environment than updates.