@Override
  public void init(ExecMapperContext context, MapredContext mrContext,
      Configuration hconf, MapJoinOperator joinOp) {
    this.tezContext = (TezContext) mrContext;
    this.hconf = hconf;
    this.desc = joinOp.getConf();
    this.cacheKey = joinOp.getCacheKey();
    String counterGroup = HiveConf.getVar(hconf, HiveConf.ConfVars.HIVE_COUNTER_GROUP);
    String vertexName = hconf.get(Operator.CONTEXT_NAME_KEY, "");
    String counterName = Utilities.getVertexCounterName(HashTableLoaderCounters.HASHTABLE_LOAD_TIME_MS.name(), vertexName);
    this.htLoadCounter = tezContext.getTezProcessorContext().getCounters().findCounter(counterGroup, counterName);
  }
HashTableLoader for Tez constructs the hashtable from records read from
a broadcast edge.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/mapjoin/fast/VectorMapJoinFastHashTableLoader.java