@Override
        public RelNode visit(ProjectQueryOperation projection) {
            List<RexNode> rexNodes = convertToRexNodes(projection.getProjectList());

            return relBuilder.project(rexNodes, asList(projection.getTableSchema().getFieldNames()), true).build();
        }
Converter from Flink's specific relational representation: {@link QueryOperation} to Calcite's specific relational
representation: {@link RelNode}.
core/src/main/java/org/apache/flink/table/planner/plan/QueryOperationConverter.java