6. The versioned relational database system set forth in claim 1 wherein: the graph is a directed acyclic graph and for a workspace that is a multi-parent workspace, the directed acyclic graph is a multi-parent graph which includes as nodes the multi-parent workspace, all of the parents of the multi-parent workspace, and all workspaces that are ancestors of the parents of the multi-parent workspace in the directed acyclic graph up through the nearest common ancestor of all of the parents, the multi-parent workspace being a leaf node of the multi-parent graph, the parents and the parents' ancestors in the directed acyclic graph up to but not including the nearest common ancestor being intermediate nodes, and the nearest common ancestor being a root node.
6. The versioned relational database system set forth in claim 1 wherein: the graph is a directed acyclic graph and for a workspace that is a multi-parent workspace, the directed acyclic graph is a multi-parent graph which includes as nodes the multi-parent workspace, all of the parents of the multi-parent workspace, and all workspaces that are ancestors of the parents of the multi-parent workspace in the directed acyclic graph up through the nearest common ancestor of all of the parents, the multi-parent workspace being a leaf node of the multi-parent graph, the parents and the parents' ancestors in the directed acyclic graph up to but not including the nearest common ancestor being intermediate nodes, and the nearest common ancestor being a root node. 7. The versioned relational database system set forth in claim 6 wherein: the processor performs operations on the versions of the data in the workspaces, the manner of performance of an operation being determined at least in part by the multi-parent graph.