Alter table to add FK constraint pegs CPU at 100% and never finishes

      In 11g2 I created a new DB using the DCA and created the schema using DDL imported into SQL Developer Data Modeler from the source database (10g2). When I created the schema I did not include the foreign-key constraints because they prevent populating the database. I populated the database via Advanced Replication. I then ran the SQL to add all the foreign-key constraints. They all happened instantly and without errors. Except one.      Adding the FK constraint from the most numerous table ("TheChild") (2.0 million rows?) referring to its parent ("TheParent") (0.5 million rows) pegged the CPU at 100% for