It is becoming increasingly important to manage databases as a
repository resource and to allow application programs to access this
resource in a heterogeneous distributed environment.  Data requested
by a query are sometimes available in multiple sites such that the
query becomes executable in different ways.  Under this circumstance,
choosing the best execution plan becomes an important task for
optimizing the query execution.  Until today, however, query
optimization in multidatabase systems has not been discussed much in
the literature.  In this work, we utilize the knowledge of scope
relationship of relations in multidatabases to identify the sites that
will return the same results.  Then, we propose a novel way of
optimizing queries which takes advantage of the conflicts of schemas
in searching for the execution plan with the least execution cost.  We
achieve the goal by first classifying various schema conflicts into
different types.  The costs of executing the same relational operation
on relations of conflicting schemas are evaluated and a weight is
assigned to each of the cases to reflect the complexity of executing
the operation.  As this method only involves simple iterative
computations of the weights and the saving of a query execution time
can be dramatic, the method developed here can be regarded as an
effective way of optimizing query processing in a multidatabase
environment.