TopNProjection topNProjection = new TopNProjection(10, 0);
node.projections(Arrays.asList(groupProjection, topNProjection));
BytesStreamOutput output = new BytesStreamOutput();
node.writeTo(output);
BytesStreamInput input = new BytesStreamInput(output.bytes());
MergeNode node2 = new MergeNode();
node2.readFrom(input);