In this paper we discuss extensions to the conventional relational algebra to support both
aspects of transaction time, evolution of a database's contents and evolution of a database's
schema. We define a relation's schema to be the relation's temporal signature, a function
mapping the relation's attribute names onto their value domains and class, indicating the
extent of support for time. We also introduce commands to change a relation, now defined as
a triple consisting of a sequence of classes, a sequence of signatures, and a sequence of …