PostgreSQL多重計數查詢性能

Question

我在postgres中編寫查詢以選擇包含更多註釋的帖子。

以下工作，但我想知道它是否會成爲許多職位的性能問題。

查詢：

SELECT 
    po.*, 
    (SELECT count(id) FROM comments WHERE post_id = po.id) AS comments_count 
    FROM posts AS po 
    ORDER BY comments_count DESC 
    LIMIT 10; 

結果：

id title body comments_count 
2 Foo Bar 5 
1 Click Bait 4 

有什麼我可以做，以提高該查詢性能還是確定？

Answer 1

您可以使用連接而不是相關的子查詢。假設ID是PK中的職位表：

select p.*, 
    count(c.id) as comments_count 
from posts p join comments c on p.id = c.post_id 
group by p.id 
order by comments_count desc limit 10; 

或

select p.*, 
    c.comments_count 
from posts p 
join (
    select post_id, 
     count(id) as comments_count 
    from comments 
    order by comments_count desc LIMIT 10 
    ) c on p.id = c.post_id;