@Override
  public void init(ExecMapperContext context, MapredContext mrContext, Configuration hconf,
      MapJoinOperator joinOp) {
    this.context = context;
    this.hconf = hconf;
    this.joinOp = joinOp;
    this.desc = joinOp.getConf();
  }
HashTableLoader for MR loads the hashtable for MapJoins from local disk (hashtables
are distributed by using the DistributedCache.
ql/src/java/org/apache/hadoop/hive/ql/exec/mr/HashTableLoader.java