1. A system that enforces currency constraints on a query on a database system having a query optimizer, a query processing engine and one or more local replicas of data in remote data sources comprising: a constraint parser that interprets and converts the currency constraint into a form usable by the query optimizer; a query plan generator that constructs a query plan for executing the query, wherein the plan includes one or more subplan selection operators that check a currency of a local replica against the query's currency constraint and selects the subplan using the local replica only when the constraint is met; and wherein the query processing engine executes the query plan, including the subplan selection operators, thereby producing a result that satisfies the query's currency constraints.