如何在RelNode中实现STRING_AGG？

import org.apache.calcite.rel.RelNode;
import org.apache.calcite.rel.RelBuilder;
import org.apache.calcite.rel.core.AggregateCall;
import org.apache.calcite.rel.core.RelFactories;
import org.apache.calcite.rel.logical.LogicalAggregate;
import org.apache.calcite.rel.logical.LogicalTableScan;
import org.apache.calcite.rex.RexInputRef;
import org.apache.calcite.rex.RexNode;
import org.apache.calcite.sql.SqlAggFunction;
import org.apache.calcite.sql.SqlKind;
import org.apache.calcite.sql.fun.SqlStdOperatorTable;
import org.apache.calcite.tools.RelBuilderConfig;
import org.apache.calcite.tools.RelBuilderFactory;

public class StringAggExample {
    public static void main(String[] args) {
        // 创建一个RelBuilder对象
        RelBuilderFactory relBuilderFactory = RelFactories.LOGICAL_BUILDER;
        RelBuilderConfig relBuilderConfig = RelBuilderConfig.DEFAULT;
        RelBuilder relBuilder = relBuilderFactory.create(relBuilderConfig);

        // 添加表扫描操作
        RelNode scan = relBuilder.scan("tableName").build();

        // 添加聚合操作
        SqlAggFunction stringAggFunction = SqlStdOperatorTable.STRING_AGG;
        RexNode column = relBuilder.field(0);
        RexNode separator = relBuilder.literal(",");
        AggregateCall aggregateCall = AggregateCall.create(stringAggFunction, false, false, false, false,
                ImmutableList.of(column, separator), -1, RelCollations.EMPTY, -1, -1);
        RelNode aggregate = relBuilder.aggregate(relBuilder.groupKey(0), ImmutableList.of(aggregateCall), scan).build();

        // 打印生成的RelNode对象
        System.out.println(aggregate);
    }
}