Hadoop二次排序关键点和出现时机（也叫辅助排序、Secondary Sort）
这样就无法将相同first的数据聚合到一个迭代中进行处理的，即相同first通过second进行排序。
?
其它pig可以通过内嵌foreach方式实现二次排序功能SQL中需要使用子查询实现该功能，见：http://heipark.iteye.com/blog/1776101?
附录下图是我整理的流程，更易于理解^_^
?
public class SecondarySort { /** * Define a pair of integers that are writable. * They are serialized in a byte comparable format. */ public static class IntPair implements WritableComparable<IntPair> { private int first = 0; private int second = 0; /** * Set the left and right values. */ public void set(int left, int right) { first = left; second = right; } public int getFirst() { return first; } public int getSecond() { return second; } /** * Read the two integers. * Encoded as: MIN_VALUE -> 0, 0 -> -MIN_VALUE, MAX_VALUE-> -1 */ @Override public void readFields(DataInput in) throws IOException { first = in.readInt() + Integer.MIN_VALUE; second = in.readInt() + Integer.MIN_VALUE; } @Override public void write(DataOutput out) throws IOException { out.writeInt(first - Integer.MIN_VALUE); out.writeInt(second - Integer.MIN_VALUE); } @Override public int hashCode() { return first * 157 + second; } @Override public boolean equals(Object right) { if (right instanceof IntPair) { IntPair r = (IntPair) right; return r.first == first && r.second == second; } else { return false; } } /** A Comparator that compares serialized IntPair. */ public static class Comparator extends WritableComparator { public Comparator() { super(IntPair.class); } public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) { return compareBytes(b1, s1, l1, b2, s2, l2); } } static { // register this comparator WritableComparator.define(IntPair.class, new Comparator()); } @Override public int compareTo(IntPair o) { if (first != o.first) { return first < o.first ? -1 : 1; } else if (second != o.second) { return second < o.second ? -1 : 1; } else { return 0; } } } /** * Partition based on the first part of the pair. */ public static class FirstPartitioner extends Partitioner<IntPair,IntWritable>{ @Override public int getPartition(IntPair key, IntWritable value, int numPartitions) { return Math.abs(key.getFirst() * 127) % numPartitions; } } /** * Compare only the first part of the pair, so that reduce is called once * for each value of the first part. */ public static class FirstGroupingComparator implements RawComparator<IntPair> { @Override public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) { return WritableComparator.compareBytes(b1, s1, Integer.SIZE/8, b2, s2, Integer.SIZE/8); } @Override public int compare(IntPair o1, IntPair o2) { int l = o1.getFirst(); int r = o2.getFirst(); return l == r ? 0 : (l < r ? -1 : 1); } } /** * Read two integers from each line and generate a key, value pair * as ((left, right), right). */ public static class MapClass extends Mapper<LongWritable, Text, IntPair, IntWritable> { private final IntPair key = new IntPair(); private final IntWritable value = new IntWritable(); @Override public void map(LongWritable inKey, Text inValue, Context context) throws IOException, InterruptedException { StringTokenizer itr = new StringTokenizer(inValue.toString()); int left = 0; int right = 0; if (itr.hasMoreTokens()) { left = Integer.parseInt(itr.nextToken()); if (itr.hasMoreTokens()) { right = Integer.parseInt(itr.nextToken()); } key.set(left, right); value.set(right); context.write(key, value); } } } /** * A reducer class that just emits the sum of the input values. */ public static class Reduce extends Reducer<IntPair, IntWritable, Text, IntWritable> { private static final Text SEPARATOR = new Text("------------------------------------------------"); private final Text first = new Text(); @Override public void reduce(IntPair key, Iterable<IntWritable> values, Context context ) throws IOException, InterruptedException { context.write(SEPARATOR, null); first.set(Integer.toString(key.getFirst())); for(IntWritable value: values) { context.write(first, value); } } } public static void main(String[] args) throws Exception { Configuration conf = new Configuration(); String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs(); if (otherArgs.length != 2) { System.err.println("Usage: secondarysort <in> <out>"); System.exit(2); } Job job = new Job(conf, "secondary sort"); job.setJarByClass(SecondarySort.class); job.setMapperClass(MapClass.class); job.setReducerClass(Reduce.class); // group and partition by the first int in the pair job.setPartitionerClass(FirstPartitioner.class); job.setGroupingComparatorClass(FirstGroupingComparator.class); // the map output is IntPair, IntWritable job.setMapOutputKeyClass(IntPair.class); job.setMapOutputValueClass(IntWritable.class); // the reduce output is Text, IntWritable job.setOutputKeyClass(Text.class); job.setOutputValueClass(IntWritable.class); FileInputFormat.addInputPath(job, new Path(otherArgs[0])); FileOutputFormat.setOutputPath(job, new Path(otherArgs[1])); System.exit(job.waitForCompletion(true) ? 0 : 1); }}?
??
--end
?