@Override
        public RexNode visit(FieldReferenceExpression fieldReference) {
            return relBuilder.field(numberOfJoinInputs, fieldReference.getInputIndex(), fieldReference.getFieldIndex());
        }
Get the {@link AggCall} correspond to the aggregate or table aggregate expression.
core/src/main/java/org/apache/flink/table/planner/plan/QueryOperationConverter.java