左联接的group_concat太慢了

Question

我有一个歌曲库和一个有效的查询，该查询连接了多个表并将所有song_id，song_title等返回到一个大表中，如下所示：

song_id             song_title          song_vocal          song_composer
--------------------------------------------------------------------------
1                   Hello Boy           John                Mike
2                   Hello Girl          Jim                 Dave
2                   Hello Girl          Tom                 Dave

我想将具有多个人声/作曲家的歌曲合并为一行：

song_id             song_title          song_vocal          song_composer
--------------------------------------------------------------------------
1                   Hello Boy           John                Mike
2                   Hello Girl          Jim,Tom             Dave

我是用group_concat（）函数完成的，但是它太慢了。 没有group_concat，我的查询只需要0.06秒。 使用此功能需要3秒钟。 这是我的查询：

SELECT
songs.song_id,
songs.title as Song_Title,
group_concat(DISTINCT a1.artist_name ORDER BY a1.artist_name DESC SEPARATOR ',' ) as Vocals,
group_concat(DISTINCT a2.artist_name ORDER BY a2.artist_name DESC SEPARATOR ',' ) as Composers    

FROM songs

left join song_vocals ON song_vocals.song_id = songs.song_id
left join artists a1 ON song_vocals.artist_id = a1.artist_id

left join song_composers on songs.song_id = song_composers.song_id
left join artists a2  on a2.artist_id = song_composers.artist_id

GROUP BY songs.song_id

我做错了什么？

Answer 1

是的，这当然太慢了。 您正在生成笛卡尔积，然后删除重复项。

我建议用相关子查询替换逻辑：

select s.song_id, s.title as Song_Title,
       (select group_concat(a.artist_name order by a.artist_name DESC separator ',' ) 
        from song_vocals sv join 
             artists a 
             on sv.artist_id = a.artist_id
        where sv.song_id = s.song_id
       ) as Vocals,
       (select group_concat(a.artist_name order by a.artist_name desc separator ',' ) 
        from song_composers sc join 
             artists a 
             on sc.artist_id = a.artist_id
        where sc.song_id = s.song_id
       ) as Composers    
from songs s;

这也可以利用song_composers(song_id, artist_id)和song_vocals(song_id, artist_id)上的索引。 我还从group_concat()删除了distinct 。 那将不再是必须的。

Answer 2

处理多个多对多关系时，几乎总是必须分别（子）查询这些关系，然后再进行合并。

SELECT s.song_id, s.title as Song_Title, v.Vocals, c.Composers    
FROM songs AS s
LEFT JOIN (
   SELECT song_id
        , GROUP_CONCAT(DISTINCT a.artist_name 
                       ORDER BY a.artist_name DESC 
                       SEPARATOR ',' 
          ) as Vocals
   FROM song_vocals AS sv
   LEFT JOIN artists AS a ON sv.artist_id = a.artist_id
   GROUP BY sv.song_id
) AS v ON s.song_id = v.song_id
LEFT JOIN (
   SELECT sc.song_id
        , GROUP_CONCAT(DISTINCT a.artist_name 
                       ORDER BY a.artist_name DESC 
                       SEPARATOR ',' 
          ) as Composers
   FROM song_composers AS sc
   LEFT JOIN artists AS a ON sc.artist_id = a.artist_id
   GROUP BY sc.song_id
) AS c ON s.song_id = c.song_id
;