sql优化

 1     # 列是表达式的一部分
 2     select * 
 3     from application as a 
 4     where a.id + 1 = 5000 
 
 5     # 列是函数的参数
 6     select * 
 7     from application as a 
 8     where date(a.created_at) = ‘20181010‘
 9  
10     # 含有隐式转换（user_id 为int类型）
11     select *
12     from application as a
13     where a.user_id = ‘567‘

1      select *
2      from application as a
3      where a.name = ‘%xia%‘
4      注：以上不能使用索引针对单列索引。对于复合索引，索引中对应列前面的列依旧可以使用索引。

 1     # 现有复合索引 cycle、sex、status
 2  
 3     select *
 4     from application as a
 5     where a.status = 2
 6       and a.sex = ‘male‘
 7     # 不符合最左前缀原则，无法使用复合索引

 8     注：针对此问题，如果字段对应值较少，可列出字段所有值，从而利用索引
 9 
10     # 现有索引 sex、cycle、status
11     select * 
12     from application as a
13     where a.sex in(‘male‘, ‘female‘)
14       and a.cycle = 2
15       and a.status = 4

 1     # 现有复合索引sex、cycle、status
 2  
 3     select *
 4     from application as a
 5     where a.sex = ‘male‘
 6       and a.cycle > 2
 7       and a.status = 3
 8     # 复合索引中的第二列使用了范围查询，第三列status无法再使用索引
 9     注：多个等值查询无此限制，即in无次限制
10       
       # 现有复合索引sex、cycle、status
11     
12     select *
13     from application as a
14     where a.sex = ‘male‘
15       and a.cycle in(3, 4)
16       and a.status = 3
17     # 使用in代替范围查询 

1         # 现有索引cycle、status
2             
3         select *
4         from application as a
5         order by a.cycle, a.status desc
6         # cycle 与 status 的排列顺序不同，无法使用索引排序

1        # 现有索引application.cycle, auto_cal_list.status
2           
3        select *
4        from application as a
5        inner jion auto_call_list as l on a.id = l.application_id
6        where a.status = 4
7        order by a.cycle, l.status
8        # order by子句有第二个

1      select *  from product where actor = ‘SE‘ and title like ‘%AP%‘
2  
3      select *  from product join(select pro_id  from product  where actor = ‘SE‘ and title like ‘%AP%‘ )as t1 on t1.prod_id = product.prod_id

1       select *
2       from application as a
3         
4       select a.id, a.user_id
5       from application as a

1      Application.filter().limit(1).execute()
2 
3      Application.filter().first()