@Override
    public Object process(Node nd, Stack<Node> stack, NodeProcessorCtx ctx, Object... nodeOutputs)
        throws SemanticException {
      Operator<?> op = (Operator<?>) nd;
      ConstantPropagateProcCtx cppCtx = (ConstantPropagateProcCtx) ctx;
      cppCtx.getOpToConstantExprs().put(op, new HashMap<ColumnInfo, ExprNodeDesc>());
      if (LOG.isDebugEnabled()) {
        LOG.debug("Stop propagate constants on op " + op.getOperatorId());
      }
      return null;
    }
The Node Processor for Constant Propagation for Operators which is designed to stop propagate.
Currently these kinds of Operators include UnionOperator and ScriptOperator.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ConstantPropagateProcFactory.java