1. A computer-implemented method executed by one or more processors, the method comprising: accessing a dataflow graph that includes deferred parallel data objects and deferred, combined parallel data operations; and executing the deferred, combined parallel operations to produce materialized parallel data objects corresponding to the deferred parallel data objects, wherein, for at least one of the deferred, combined parallel operations, executing includes: determining an estimated size of data associated with the at least one deferred, combined parallel operation; determine whether the estimated size exceeds a threshold size; if the estimated size is below a threshold size, execute the at least one deferred, combined parallel operation as a local, sequential operation; and if the estimated size exceeds a threshold size, execute the at least one deferred, combined parallel operation as remote, parallel operation.