1. A method for protecting privacy data in a MapReduce system comprising a private subsystem and a public subsystem, the method comprising: receiving a MapReduce program to be executed by a computer processor in the MapReduce system, wherein the MapReduce program contains instructions that designate a privacy property of input data and output data of a Map function and output data of a Reduce function; detecting the privacy property of the input and output data of the Map function and the output data of the Reduce function prior to execution of the MapReduce program; assigning the data for processing by the private subsystem and the public subsystem based on the detected privacy property, wherein the data of which the privacy property is private are assigned to the private subsystem; dynamically detecting the privacy property of actual output data of the Map function and the Reduce function while the MapReduce program is running; and assigning the actual output data appropriately to the public subsystem of the private subsystem according to the privacy property of the actual output data, wherein the MapReduce program comprises extended instructions for reporting the privacy property of the actual output data of the Map function and the privacy property of the actual output data of the Reduce function, and the dynamically detecting the privacy property of the actual output data of the Map function and the Reduce function while the MapReduce program is running further comprises examining the privacy property of the actual output data of the Map function and the Reduce function according to a report in the MapReduce program runtime.