1. A method comprising the steps of receiving a computer programming language database system query; generating at least one relational algebraic expression in response to said query by employing at least one of a collection of equivalence rules involving the multiset version of the relational algebraic theta-semijoin operator; selecting at least one of said relational algebraic expressions; and accessing said computer programming language database based on said selected expression.
2. The method of claim 1 wherein said selecting comprises the step of estimating processing costs for at least one of said expressions.