19. The versioned relational database system set forth in claim 7 wherein: an operation that operates on a workspace that belongs to a multi-parent graph or on a version in such a workspace and that may result in a constraint violation uses the multi-parent graph to determine whether the operation will violate the constraint.
22. The versioned relational database system set forth in claim 19 wherein: the constraint is a referential integrity constraint, the referential integrity constraint being violated when the operation results in a multi-parent graph such that a current version of a first table in a first workspace belonging to the multi-parent graph violates a referential integrity constraint with regard to a current version of a second table in a second workspace that belongs to the multi-parent graph and is a descendant of the first workspace.