code: public void setAndLinkChild(int index, AbstractPlanNode child) {
          assert(child != null);
          m_children.set(index, child);
          child.m_parents.add(this);
      }
docs: Used to re-link the child without changing the order.
  
  This is called by PushDownLimit and RemoveUnnecessaryProjectNodes.
  @param index
  @param child