Schema update operations preserving the expressive power in XML databases

This paper proposes schema update operations preserving the expressive power in XML databases. XML documents are modeled as labeled ordered trees and schemas are modeled as regular tree grammars. Under these models, the notion of embeddability between tree languages is defined. We say that a schema G_2 has more expressive power than G_1 if the tree language generated by G_1 is embeddable in that generated by G-2. Then, four update operations on schemas are introduced. It is shown that the operations are sound with respect to preservation of the expressive power. Moreover, it is shown that the operations are also complete for local tree grammars, which correspond to DTDs.