@Override
    public Object process(Node nd, Stack<Node> stack, NodeProcessorCtx ctx,
        Object... nodeOutputs) throws SemanticException {
      JoinOperator op = (JoinOperator) nd;
      pruneJoinOperator(ctx, op, op.getConf(), op.getColumnExprMap(), null,
          false);
      return null;
    }
The Node Processor for Column Pruning on Join Operators.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ColumnPrunerProcFactory.java