presto-parquet/src/main/java/io/prestosql/parquet/ParquetTypeUtils.java/ParquetTypeUtils/getMapKeyValueColumn
class ParquetTypeUtils:    public static GroupColumnIO getMapKeyValueColumn(GroupColumnIO groupColumnIO)
    {
        while (groupColumnIO.getChildrenCount() == 1) {
            groupColumnIO = (GroupColumnIO) groupColumnIO.getChild(0);
        }
        return groupColumnIO;
    }