大查询和子查询的问题

Question

我认为我会聪明并使用子查询一次性获取我的报告.但在遇到问题并阅读文档后,我发现我的方法在MySQL中不起作用.我的内部查询返回~100条记录,外部查询扫描20000条记录.当我将外部查询限制为20条记录时,它运行20秒 - 非常慢.

我想知道是否有可能以某种方式重构它,以便外部查询中的每个记录都不会运行内部查询？

select p1.surname ,p1.name,p1.id,r1.start_date,r1.end_date,c1.short_name
FROM ejl_players p1
left JOIN ejl_registration r1 ON ( r1.player_id = p1.id )
left JOIN ejl_teams t1 ON ( r1.team_id = t1.id )
left JOIN ejl_clubs c1 ON ( t1.club_id = c1.id )
where  r1.season=2008
and p1.id in
 (
SELECT p.id
FROM ejl_players p 
left JOIN ejl_registration r ON (r.player_id = p.id) 
left JOIN ejl_teams t ON (r.team_id = t.id) 
left JOIN ejl_clubs c ON (t.club_id = c.id)
WHERE r.season = 2008
GROUP BY p.id
HAVING COUNT(DISTINCT c.id)  > 1
)

解释(我将外部查询限制为最多20条记录:

id  select_type  table  type  possible_keys  key  key_len  ref  rows  Extra  
1 PRIMARY p1 range PRIMARY PRIMARY 4 NULL 19 Using where 
1 PRIMARY r1 ref team_id,season season 10 const,d17528sd14898.p1.id 1 Using where 
1 PRIMARY t1 eq_ref PRIMARY PRIMARY 4 d17528sd14898.r1.team_id 1   
1 PRIMARY c1 eq_ref PRIMARY PRIMARY 4 d17528sd14898.t1.club_id 1   
2 DEPENDENT SUBQUERY p index PRIMARY PRIMARY 5 NULL 23395 Using index 
2 DEPENDENT SUBQUERY r ref team_id,season season 10 const,d17528sd14898.p.id 1 Using where; Using index 
2 DEPENDENT SUBQUERY t eq_ref PRIMARY PRIMARY 4 d17528sd14898.r.team_id 1   
2 DEPENDENT SUBQUERY c eq_ref PRIMARY PRIMARY 4 d17528sd14898.t.club_id 1 Using index 

Answer 1

尝试使用INNER JOIN(类似这样):

SELECT p1.surname ,p1.name,p1.id,r1.start_date,r1.end_date,c1.short_name
FROM ejl_players p1
INNER JOIN (
    SELECT p.id
    FROM ejl_players p 
    LEFT JOIN ejl_registration r ON (r.player_id = p.id) 
    LEFT JOIN ejl_teams t ON (r.team_id = t.id) 
    LEFT JOIN ejl_clubs c ON (t.club_id = c.id)
    WHERE r.season = 2008
    GROUP BY p.id
    HAVING COUNT(DISTINCT c.id)  > 1
) p2 ON p1.id = p2.id
LEFT JOIN ejl_registration r1 ON ( r1.player_id = p1.id )
LEFT JOIN ejl_teams t1 ON ( r1.team_id = t1.id )
LEFT JOIN ejl_clubs c1 ON ( t1.club_id = c1.id )
WHERE  r1.season=2008

以这种方式使用子查询应该更有效但并非总是如此.但是,它确实绕过了为主查询中返回的每个记录执行子查询的问题.而是将子查询构造为内存中的虚拟表,然后用于与主查询进行比较.

编辑:我应该指出,您将要在MySQL中使用EXPLAIN来验证此查询确实更有效地执行.