问题

select mu.* from mission_user mu left join  mission_daily md on mu.`user_id`=md.user_id and md.mission_id=mu.mission_id 
            left join mission_weekly mw on mu.`user_id`=mw.user_id and mw.mission_id=mu.mission_id 
            where (md.day='2019-02-21' and md.status='GOING' and md.`user_id`=478) or (mw.year_week=201909 and mw.status='GOING' and mw.`user_id`=478) 
             limit 0,20
//执行时间:10.2s

• 执行时间 10.2s

排查,各去除一个or 条件

select mu.* from mission_user mu left join  mission_daily md on mu.`user_id`=md.user_id and md.mission_id=mu.mission_id 
            left join mission_weekly mw on mu.`user_id`=mw.user_id and mw.mission_id=mu.mission_id 
            where (mw.year_week=201909 and mw.status='GOING' and mw.`user_id`=478) 
             limit 0,20
//执行时间:35ms

select mu.* from mission_user mu left join  mission_daily md on mu.`user_id`=md.user_id and md.mission_id=mu.mission_id 
            left join mission_weekly mw on mu.`user_id`=mw.user_id and mw.mission_id=mu.mission_id 
            where (md.day='2019-02-21' and md.status='GOING' and md.`user_id`=478)  
             limit 0,20
//执行时间:32ms

慢原因分析

由于为对主表加条件导致，mission_user 整表查询出来在left join 导致缓慢.

解决方法，加入mission_user 条件到where

select mu.* from mission_user mu left join  mission_daily md on mu.`user_id`=md.user_id and md.mission_id=mu.mission_id 
            left join mission_weekly mw on mu.`user_id`=mw.user_id and mw.mission_id=mu.mission_id 
            where mu.user_id=478 and ((md.day='2019-02-21' and md.status='GOING' and md.`user_id`=478) or (mw.year_week=201909 and mw.status='GOING' and mw.`user_id`=478) )
             limit 0,20
//执行时间:116ms