Oracle7 Server Application Developer's Guide
Referential Integrity in a Distributed Database
Oracle does not permit declarative referential integrity constraints to be defined across nodes of a distributed database (that is, a declarative referential integrity constraint on one table cannot specify a foreign key that references a primary or unique key of a remote table). However, parent/child table relationships across nodes can be maintained using triggers. For more information about triggers that enforce referential integrity, refer to Chapter 9. Using triggers to maintain referential integrity requires the distributed option; for more information refer to Oracle7 Server Distributed Systems, Volume I.
Note: If you decide to define referential integrity across the nodes of a distributed database using triggers, be aware that network failures can limit the accessibility of not only the parent table, but also the child table. For example, assume that the child table is in the SALES database and the parent table is in the HQ database. If the network connection between the two databases fails, some DML statements against the child table (those that insert rows into the child table or update a foreign key value in the child table) cannot proceed because the referential integrity triggers must have access to the parent table in the HQ database.