使用 explain 如何判断二级索引使用后是否回表？

select
    track_source_id,
    date_format(created_at, '%y-%m-%d') as day,
    count(*) as total_count,
    sum(case when len_parse_result_list = 0 then 1 else 0 end) as len_parse_result_list_zero_count,
    sum(case when len_parse_result_list is null then 1 else 0 end) as len_parse_result_list_null_count,
    sum(case when len_parse_result_list > 0 then 1 else 0 end) as len_parse_result_list_gte_zero_count
from
    keywordtask
where
    created_at >= now() - interval 30 day
group by
    track_source_id,
    day
order by
    track_source_id,
    day;

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
| --- | ----------- | ----- | ---------- | ---- | ------------- | --- | ------ | --- | ---- | -------- | ----- |
| 1   | SIMPLE      | keywordtask | NULL      | index | idx_created_at,idx_track_source_id_created_at_len_parse_result_list | idx_track_source_id_created_at_len_parse_result_list | 14 | NULL | 134324154 | 50.0 | Using where; Using index; Using temporary; Using filesort |