@Override
            public RelBuilder.AggCall visit(CallExpression call) {
                SqlAggFunction sqlAggFunction = call.accept(sqlAggFunctionVisitor);
                return relBuilder.aggregateCall(
                        sqlAggFunction,
                        false,
                        false,
                        null,
                        sqlAggFunction.toString(),
                        call.getChildren().stream().map(expr -> expr.accept(expressionConverter)).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