1. A method of altering an existing data structure in a database system, said existing data structure including a plurality of related tables of columns and rows of data values and further including one or more indexes for said related tables to provide a relational database, said existing data structure being defined by a catalog data structure including tables of tables and indexes, said method comprising the steps of: a) receiving a change indication signal, 1) said change indication signal encoding one or more statements conforming to a change definition language that is a superset of a SQL structured query language, 2) said change definition language including a plurality of available statements for changing at least one of A) a data specification of a column, B) an owner of a table or an index, and C) a name of a column or an index, 3) said one or more statements specifying one or more requested changes to be made in the existing data structure; b) testing whether said one or more requested changes can be made in the existing data structure; and c) if said changes can be made in said existing data structure, then making said changes, or else signaling an error.