如何加速從兩個表中搜索的sql查詢？

Question

下面是一個例子，所以我有表user和表city，他們是這樣的：

user列（USER_ID，city_id，時間戳）[USER_ID和city_id是獨一無二]

city列是（CITY_NAME，city_id）[city_id是獨一無二]

我想獲得用戶數量從某城市一個給定的日期，所以基本上我做了這樣的：

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 

然後，我意識到這是不是直接用於

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

這是爲什麼搜索方式慢？原始搜索中是不是ct.city_id已被設置爲20或30？我應該如何優化搜索並以我想要的表格格式獲得結果？

Answer 1

你可以提高你的查詢，避免子查詢和使用內部連接和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

我會假設，MySQL是選擇到您的派生表物化到一個內部臨時表中的第一個查詢，不會在第二個查詢中做出該選擇。

對於派生表（子查詢在FROM子句）時，優化這些選擇：

• 合併派生表到外部查詢塊
• 物化爲派生表到內部臨時表

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

Answer 3

試試這個：

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'