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 in the MapReduce system, wherein the MapReduce program contains instructions that designate the privacy property of the input and output data of a Map function and the 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; and 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.