Multidatabase querying is based on the assumption that each database in the multidatabase system provides access for ad hoc queries. In addition, constructing a multidatabase system requires detailed documentation of the schemas and semantics of each component database, including information on links to other databases and semantic heterogeneity problems.
A variety of problems were encountered in performing multidatabase
queries with OPM*QS, including semantic heterogeneity problems that
had to be resolved. For example accession numbers are represented
in different formats in different databases: a GDB accession number
``GDB:118746'' is represented as ``G00-118-746''
in GSDB 2.0, while a GSDB accession number ``L05367'' is
represented in GDB as ``GSDB:L05367''. OPM*QL supports some
general string manipulation and comparison operators in order to
resolve such problems. It is hoped that such incompatibilities
will normally be hidden in the code for inter-database links, and will
therefore not concern users performing multidatabase queries. However
merely identifying such incompatibilities, and determining how to
resolve them, is a difficult problem, in part because of the lack of
detailed documentation for individual database schemas.