for(Ordering od : popConfig.getOrderings()){
// first, we rewrite the evaluation stack for each side of the comparison.
ErrorCollector collector = new ErrorCollectorImpl();
final LogicalExpression expr = ExpressionTreeMaterializer.materialize(od.getExpr(), batch, collector,context.getFunctionRegistry());
if(collector.hasErrors()) throw new SchemaChangeException("Failure while materializing expression. " + collector.toErrorString());
g.setMappingSet(LEFT_MAPPING);
HoldingContainer left = g.addExpr(expr, false);
g.setMappingSet(RIGHT_MAPPING);
HoldingContainer right = g.addExpr(expr, false);
g.setMappingSet(MAIN_MAPPING);