public void reduce(ImmutableBytesWritable key, Iterator<Put> values,
    OutputCollector<ImmutableBytesWritable, Put> output, Reporter reporter) throws IOException {

    while (values.hasNext()) {
      output.collect(key, values.next());
    }
  }
No aggregation, output pairs of (key, record)
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapred/IdentityTableReduce.java