1. A computer-implemented method, comprising: executing a deferred, combined parallel operation, which is included in a dataflow graph that comprises deferred parallel data objects and deferred, combined parallel operations corresponding to a data parallel pipeline, to produce materialized parallel data objects corresponding to deferred parallel data objects, wherein the executing comprises: determining an estimated size of data associated with the deferred, combined parallel operation being executed; determining that the estimated size of data associated with the deferred, combined parallel operation exceeds a threshold size based at least on accessing annotations in the dataflow graph that represent an estimate of the size of the data associated with the deferred, combined parallel operation; and in response to determining that the estimated size exceeds the threshold size, executing the deferred, combined parallel operation as a remote, parallel operation.