## org.apache.hadoop.mapred.join
Class OverrideRecordReader<K extends WritableComparable,V extends Writable>

    java.lang.Object
      org.apache.hadoop.mapred.join.CompositeRecordReader<K,V,V>
          org.apache.hadoop.mapred.join.MultiFilterRecordReader<K,V>
              org.apache.hadoop.mapred.join.OverrideRecordReader<K,V>

All Implemented Interfaces:
  Comparable<ComposableRecordReader<K,?>>, Configurable, ComposableRecordReader<K,V>, RecordReader<K,V>

    public class OverrideRecordReader<K extends WritableComparable,V extends Writable>

extends MultiFilterRecordReader<K,V>

Prefer the "rightmost" data source for this key. For example, `override(S1,S2,S3)` will prefer values from S3 over S2, and values from S2 over S1 for all keys emitted from all sources.

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.hadoop.mapred.join.MultiFilterRecordReader
`MultiFilterRecordReader.MultiFilterDelegationIterator`

Field Summary

Fields inherited from class org.apache.hadoop.mapred.join.CompositeRecordReader
`jc, kids`

Method Summary
`protected V` `emit(TupleWritable dst)` Emit the value with the highest position in the tuple.
`protected void` `fillJoinCollector(K iterkey)` Instead of filling the JoinCollector with iterators from all data sources, fill only the rightmost for this key.

Methods inherited from class org.apache.hadoop.mapred.join.MultiFilterRecordReader
`combine, createValue, getDelegate, next`

Methods inherited from class org.apache.hadoop.mapred.join.CompositeRecordReader
`accept, add, close, compareTo, createInternalValue, createKey, getComparator, getConf, getPos, getProgress, getRecordReaderQueue, hasNext, id, key, key, setConf, skip`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Methods inherited from interface org.apache.hadoop.mapred.join.ComposableRecordReader
`accept, hasNext, id, key, key, skip`

Methods inherited from interface org.apache.hadoop.mapred.RecordReader
`close, createKey, getPos, getProgress`

Methods inherited from interface java.lang.Comparable
`compareTo`

Method Detail

### emit

    protected V emit(TupleWritable dst)

  Emit the value with the highest position in the tuple.

  Specified by:
    `emit` in class `MultiFilterRecordReader<K extends WritableComparable,V extends Writable>`

### fillJoinCollector

    protected void fillJoinCollector(K iterkey)
                              throws IOException

  Instead of filling the JoinCollector with iterators from all data sources, fill only the rightmost for this key. This not only saves space by discarding the other sources, but it also emits the number of key-value pairs in the preferred RecordReader instead of repeating that stream n times, where n is the cardinality of the cross product of the discarded streams for the given key.

  Overrides:
    `fillJoinCollector` in class `CompositeRecordReader<K extends WritableComparable,V extends Writable,V extends Writable>`
  Throws:
    `IOException`

Copyright © 2009 The Apache Software Foundation