private List<RexNode> convertToRexNodes(List<ResolvedExpression> expressions) {
            return expressions
                    .stream()
                    .map(QueryOperationConverter.this::convertExprToRexNode)
                    .collect(toList());
        }
Get the {@link AggCall} correspond to the aggregate or table aggregate expression.
core/src/main/java/org/apache/flink/table/planner/plan/QueryOperationConverter.java