1. A system comprising: one or more processing devices; one or more storage devices, the storage devices storing instructions that, when executed by the one or more processing devices, implement the following: an application that includes a data parallel pipeline, wherein the data parallel pipeline specifies multiple parallel data objects that contain multiple elements and multiple parallel operations that operate on the parallel data objects; an evaluator configured, based on the data parallel pipeline, to generate a dataflow graph of deferred parallel data objects and deferred parallel operations corresponding to the data parallel pipeline; an optimizer configured to apply one or more graph transformations to the dataflow graph to generate a revised dataflow graph that includes one or more of the deferred parallel data objects and deferred, combined parallel data operations; and an executor configured to execute the deferred, combined parallel operations to produce materialized parallel data objects corresponding to the deferred parallel data objects.