Schema evolution and the relational algebra

摘要

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 states. A semantic type of system is required to identify semantically incorrect expressions and to enforce consistency constraints among a relation's class, signature and state following update. We show that these extensions are applicable, without change, to historical algebras that support valid time, yielding an algebraic language for the query and update of temporal databases. The additions preserve the useful properties of the conventional algebra.