for (int i = 0; i < this.getInputs().size(); i++) {
inputPops.add( ((Prel)this.getInputs().get(i)).getPhysicalOperator(creator));
}
///TODO: change this to UnionDistinct once implemented end-to-end..
UnionAll unionAll = new UnionAll(inputPops.toArray(new PhysicalOperator[inputPops.size()]));
return creator.addMetadata(this, unionAll);
}