1. A computer-implemented method for processing a program statement in a database query language, the program statement corresponding to a plurality of operators, wherein an operator tree is associated with the plurality of operators, the operator tree comprising a parent operator node, the method comprising: identifying a child node that is associated with the parent operator node; determining if the child node relates to an operator for which top-down processing is capable of being performed, wherein the top-down processing is capable of being performed when a result for the operator is capable of being generated without storage of the result for the parent operator node; calling and executing the operator for the child node to generate a result using a processor; and outputting the result to a data stream without buffering the result or an intermediate result in storage when top-down processing is performed, thereby processing the program statement in the database query language.