A computer is programmed to accept queries over streams of, data structured as per a predetermined syntax (e.g. defined in XML). The computer is further programmed to execute such queries continually (or periodically) on data streams of tuples containing structured data that conform to the same predetermined syntax. In many embodiments, the computer includes an engine that exclusively processes only structured data, quickly and efficiently. The computer invokes the structured data engine in two different ways depending on the embodiment: (a) directly on encountering a structured data operator, or (b) indirectly by parsing operands within the structured data operator which contain path expressions, creating a new source to supply scalar data extracted from structured data, and generating additional trees of operators that are natively supported, followed by invoking the structured data engine only when the structured data operator in the query cannot be fully implemented by natively supported operators.