@Explain(displayName = "keyExpressions", explainLevels = { Level.DEBUG })
  public Map<Byte, String> getKeyExpressionString() {
    Map<Byte, String> keyMap = new LinkedHashMap<Byte, String>();
    for (Map.Entry<Byte, List<ExprNodeDesc>> k: getKeys().entrySet()) {
      keyMap.put(k.getKey(), k.getValue().toString());
    }
    return keyMap;
  }
@param dumpFilePrefix
         the dumpFilePrefix to set
ql/src/java/org/apache/hadoop/hive/ql/plan/MapJoinDesc.java