1. A method for processing queries comprising: executing a query to generate a relation defined by a model clause specified in said query, said model clause specifying: one or more dimension columns of said relation, and a rule, wherein said rule comprises: a left-side expression, wherein said left-side expression includes one or more existential predicates, wherein each of said one or more existential predicates evaluates to a Boolean value and refers to at least one of said one or more dimension columns of said relation; and a right-side expression; wherein executing said query comprises evaluating said rule, wherein evaluating said rule includes performing an UPSERT operation to insert a row into said relation; and wherein the method is performed by one or more computer systems.