PhysicalOperator childPOP = child.getPhysicalOperator(creator);
if(PrelUtil.getSettings(getCluster()).isSingleMode()) return childPOP;
SingleMergeExchange g = new SingleMergeExchange(childPOP, PrelUtil.getOrdering(this.collation, getChild().getRowType()));
return creator.addMetadata(this, g);
}