mysql - 如何进行更快的 best-n-per-group 查询？

Question

我正在使用这个查询：

SELECT district, id
FROM adverts ls
GROUP BY district, id
HAVING (

SELECT count( * )
FROM adverts
WHERE district = ls.district
AND id > ls.id
) <5
ORDER BY district, id DESC ;

LIMIT 0 , 30

大约需要 35 秒。这是解释：

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   PRIMARY     ls  range   NULL    i_id    5   NULL    16166   Using index for group-by; Using temporary; Using f...
2   DEPENDENT SUBQUERY  ilan_genel  ref     PRIMARY,i_id,i_ozellik_id,i_tip_fiyat,i_tip_id,i_d...   i_durum_id  2   func    25  Using where; Using index 

有没有办法让它更快？

Answer 1

试试这个：

SELECT    district, id, COUNT(b.district)
FROM      adverts a INNER JOIN adverts b
              ON a.district = b.district
WHERE     b.id > a.id
GROUP BY  district, id 
HAVING    COUNT(b.district) < 5
ORDER BY  district, id DESC 

根据定义，Joins比快。subqueries