如何加速从两个表中搜索的sql查询？

Question

下面是一个例子，所以我有表user和表city，他们是这样的：

user列（USER_ID，city_id，时间戳）[USER_ID和city_id是独一无二]

city列是（CITY_NAME，city_id）[city_id是独一无二]

我想获得用户数量从某城市一个给定的日期，所以基本上我做了这样的：

select city_id, city_name, 
    (select count(user.user_id) 
    from user, city 
    where DATE_FORMAT(user.timestamp, '%Y-%m-%d') = '2017-03-07' 
    and user.city_id = ct.city_id) as user_count 
from city ct 
where (city_id = 20 or city_id = 30) 

和Result：

city_id, city_name, user_count 
20  New York 100 
30  LA   200 

然后，我意识到这是不是直接用于

select count(user.user_id) 
from user, city 
where DATE_FORMAT(user.timestamp, '%Y-%m-%d') = '2017-03-07' 
    and user.city_id = 20 

这是为什么搜索方式慢？原始搜索中是不是ct.city_id已被设置为20或30？我应该如何优化搜索并以我想要的表格格式获得结果？

Answer 1

你可以提高你的查询，避免子查询和使用内部连接和group by

select city_id, city_name, count(user.user_id) 
from user 
inner join city on user.city_id = city.city_id 
where DATE_FORMAT(user.timestamp, '%Y-%m-%d') = '2017-03-07' 
and city_id in (city_id = 20 or city_id = 30) 
group by city_id, city_name 

Answer 2

我会假设，MySQL是选择到您的派生表物化到一个内部临时表中的第一个查询，不会在第二个查询中做出该选择。

对于派生表（子查询在FROM子句）时，优化这些选择：

• 合并派生表到外部查询块
• 物化为派生表到内部临时表

源：Mysql Documentation - 8.2.2 Optimizing Subqueries, Derived Tables, and Views

Answer 3

试试这个：

select city_id, city_name, count(user.user_id) as user_count 
from city ct 
inner join user on user.city_id = ct.city_id 
where (ct.city_id = 20 or ct.city_id = 30) 
AND DATE_FORMAT(user.timestamp, '%Y-%m-%d') = '2017-03-07'