为什么即使子查询执行得很快，带子查询的SQL查询还是这么慢？

Question

My query is something like:

SELECT *,
(SELECT COUNT(*) FROM comments WHERE comments.thread = threads.id) AS comments
FROM threads
LIMIT 10

comments.thread is an index, queries like this run fast:

SELECT COUNT(*) FROM comments WHERE comments.thread = 'someId'

However, my query is extremly slow. It takes 10 seconds times the limit I define. Why?

Answer 1

For this query:

SELECT t.*,
      (SELECT COUNT(*) FROM comments c WHERE c.thread = t.id) AS comments
FROM threads t
LIMIT 10;

You want an index on comments(thread) . If your other query runs fast, then I would guess that you already have one.

Perhaps the LIMIT and subquery are acting strangely. Is this version also slow?

SELECT t.*,
      (SELECT COUNT(*) FROM comments c WHERE c.thread = t.id) AS comments
FROM (SELECT t.*
      FROM threads t
      LIMIT 10
     ) t;

Answer 2

Your inner query is a corelated subquery, meaning it uses a value from the outer query, so executes for every row of the outer query. Maybe MySQL is not so good at optimizing the query.

Try this:

SELECT threads.*, count(comments.thread) AS comments
FROM threads
JOIN comments ON comments.thread = threads.id
GROUP BY 1,2,3,4,5 -- one number here for each column of the threads table
LIMIT 10