Is job.setNumReduceTasks(0) equivalent to having a blank reduce function

Question

job.setNumReduceTasks(0) results in a map only job

does this mean intermediate phase (shuffle and sort) are not performed?

how is it compared to having an empty reduce method (no operations):

public static class Reduce extends MapReduceBase implements Reducer<Text, IntWritable, Text, IntWritable> {
  public void reduce(Text key, Iterator<IntWritable> values, OutputCollector<Text, IntWritable> output, Reporter reporter) throws IOException {                             
    \\do nothing            
  }
}

Or is it equivalent

Answer 1

The difference is simple,

1. Map-Only Job In a Map-only job, you do not have shuffle phase, which means no data is sending across the network. The mappers will generate automatically the results. Check this out.
2. Map-Reduce Job Even though your reducers are doing nothing, the data will be sent to the reducers, which means, shuffle phase is happening. Reducers will write the results to disk.