1. A computer-implemented method for use in evaluating queries, the method comprising: receiving a query that includes an expression; determining whether a part of the expression in the query is equivalent to a defining expression for a virtual column of a table; wherein database metadata defines said virtual column as a column of the table that contains, for each row in said table, a data value computed from the defining expression; wherein said database metadata includes a name for said virtual column that is used in queries to reference said virtual column; in response to determining that the part of the expression is equivalent to the defining expression for the virtual column, rewriting the query to generate a rewritten query, wherein the step of rewriting the query comprises replacing, in the query, the part of the expression with the virtual column; and processing the rewritten query; wherein the method is performed by one or more computing devices.