Path expressions, a c e n tral ingredient of query languages for objectoriented databases, are currently used as a purely navigational vehicle. We argue that this does not fully exploit their potential expressive p o wer as a tool to specify connections between objects. In particular, a user should not be required to specify a path to be followed in full, but rather should provide enough information so that the system can infer missing details automatically. We present and study an extended

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... anism for path expressions which resembles the omission of joins in universal relation interfaces. The semantics of our mechanism is given in the general framework of a calculus-like query language. Techniques from semantic query optimization are employed to obtain e cient speci cations. We also consider the possibility that links can be traversed backwards, which subsumes previous proposals to specify inverse relationships at the schema level and also fully exploits the meaning of inheritance links. : PistonEngine hp: Integer cc: Integer corresponding to our running example will include, among others: Person(Person) Person-name(Person, name) Employee(Employee) Employee-familyMembers(Employee, familyMembers) Company(Company) Company-president(Company, president). The constraints are functional, inclusion and exclusion dependencies 6], used to model the obvious properties of single-valued attributes, inheritance, referential integrity, and disjointness of classes unrelated in the inheritance hierarchy in our running example, these will include, among others: Employee Employee] Person Person] Company-president president] Employee Employee] Vehicle-manufacturer manufacturer] Company Company] Company Company] \ Person Person] = Each given OODB schema can uniquely be represented in relational terms in this way. Hence, instances over the OODB schema can simply be introduced as