问两种1:N分组方法的性能

EN

class ParentDto {
    public Long id;
    public String someColumn;
    public List<ChildDto> children = new ArrayList<>();
}

class ChildDto {
    public Long id;
    public Long parentId;
    public String someColumn;
}

SELECT * 
FROM PARENT_DTO 
    JOIN CHILD_DTO ON PARENT_DTO.ID = CHILD_DTO.PARENT_ID

// assume that query is some api to run a query like jdbc that returns theortical type List<Row>
List<ParentDto> results = query(/* query above*/).stream()
    // assume that first value in row is ParentDto::id
    .collect(groupingBy(row -> (Long) row.get(0)))
    // after collect stream type is Map<Long, List<Row>>, whatever row type is from whatever api
    .values()
    .stream()
    // listOfGroupRows is just List<Row>
    .map(listOfGroupedRows -> {
        // assume that ParentDto constructor knows how to fetch values from first row 
        ParentDto rowValue = new ParentDto(listOfGroupedRows.get(0))
        // assume that ChildDto constructor knows how to fetch values from each row
        rowValue.children = listOfGroupedRows.stream()
            .map(row -> new ChildDto(row))
            .collect(toList())
    })

SELECT *
FROM PARENT_DTO

SELECT *
FROM CHILD_DTO 

Map<Long, List<ChildDto>> children = query(/* second query */).stream()
    .map(r -> new ChildDto(r))
    .collect(groupingBy(ChildDto::parentId));

List<ParentDto> results = query(/* first query */).stream()
    .map(r -> {
        ParentDto dto = new ParentDto(r);
        dto.children = children.get(dto.id);
        return dto;
    })
    .collect(toList());