这个 sql 如何优化？

Question

下面这个 sql，ci_customer 表有 40w 数据，ci_follow 表有300w 数据，查询速度为 10s。请问是否有方法将这个 sql 优化一下呢。

f.customer_id、f.id、c.id 都是有索引的。

SELECT COUNT(*) AS `numrows`
FROM (`ci_follow` as f)
right join ci_customer as c on c.id = f.customer_id
WHERE (1 = 1)
  and f.isdel = 0
  and c.isdel = 0
order by f.id desc;

explain如下