public JoinUtil.JoinResult setFromOutput(Output output) {

      aliasFilter = hashMap.getValueResult(
              output.getData(), 0, output.getLength(), hashMapResult, /* matchTracker */ null);
      dummyRow = null;
      wasFirstCalledOnDummyRow = false;
      if (hashMapResult.hasRows()) {
        return JoinUtil.JoinResult.MATCH;
      } else {
        aliasFilter = (byte) 0xff;
        return JoinUtil.JoinResult.NOMATCH;
      }
    }
Sometimes, when container is empty in multi-table mapjoin, we need to add a dummy row.
This container does not normally support adding rows; this is for the dummy row.
ql/src/java/org/apache/hadoop/hive/ql/exec/persistence/MapJoinBytesTableContainer.java