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Object-oriented databases enforce behavioral
schema consistency rules to guarantee type
safety, i.e., that no run-time type error can
occur. When the schema must evolve, some
schema updates may violate these rules. In
order to maintain complete behavioral schema
consistency, traditional solutions require significant
changes to the types, the type hierarchy
and the code of existing methods. Such
operations are very expensive in a database
context. To ease schema evolution, we propose
to support exceptions to the behavioral
consistency rules without sacrificing type
safety for all that. The basic idea is to detect
unsafe statements at compile-time and
check them at run-time. The run-time check
is performed by a specific clause that is
automatically inserted around unsafe statements.
This check clause warns the programmer
of the safety problem and lets him provide
exception-handling code. Schema updates
can therefore be performed with only
minor changes to the code of methods.