How to improve performance this sql of mysql

Question

I have two tables, and now I must order table's A by B 's count：

A：

id  | name      | status
--- |-----------|---------
1   |  test     |  1
2   |  this is 2|  1

B：

id | a_id | user_id
---|------|----------
1  | 1    |   12
2  | 1    |   13
3  | 1    |   14
4  | 2    |   12
5  | 2    |   15

Now I want to fetch limit 5 of the most users from table A :

SELECT a.name, b.total
FROM tb_a a
INNER JOIN (SELECT COUNT(*) total, a_id FROM tb_b GROUP BY a_id) b
ON a.id = b.a_id
WHERE a.status = 1
ORDER BY b.`total` DESC

I have created index for a_id column in table B .

After executed, explain :

*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: a
         type: ALL
possible_keys: PRIMARY
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 2
        Extra: Using where; Using temporary; Using filesort
*************************** 2. row ***************************
           id: 1
  select_type: PRIMARY
        table: <derived2>
         type: ref
possible_keys: <auto_key0>
          key: <auto_key0>
      key_len: 4
          ref: test.a.id
         rows: 2
        Extra: NULL
*************************** 3. row ***************************
           id: 2
  select_type: DERIVED
        table: tb_b
         type: index
possible_keys: idx_id
          key: idx_id
      key_len: 4
          ref: NULL
         rows: 7
        Extra: Using index
3 rows in set (0.00 sec)

I find that the first used Using temporary; Using filesort , I know this must be refactored!

But how can I do it?

Answer 1

Why do you need subquery?

Won't this work for you:

SELECT a.name, COUNT(b.id) as total
FROM tb_a a
INNER JOIN tb_b b ON a.id = b.a_id
WHERE a.status = 1
GROUP BY a.id
ORDER BY COUNT(b.id) DESC