@Override
  public void startGroup() throws HiveException {
    lastChildIndex = 0;
    super.startGroup();
  }
We assume that the input rows associated with the same key are ordered by
the tag. Because a tag maps to a childindex, when we see a new childIndex,
we will not see the last childIndex (lastChildIndex) again before we start
a new key group. So, we can call flush the buffer of children
from lastChildIndex (inclusive) to currentChildIndex (exclusive) and
propagate processGroup to those children.
@param currentChildIndex the childIndex we have right now.
@throws HiveException
ql/src/java/org/apache/hadoop/hive/ql/exec/DemuxOperator.java