MySQL 子查询慢

Question

以下查询需要1-2秒查询。

SELECT updated, COUNT( * ) count
FROM v2_subscription
WHERE ss_id IN (SELECT MAX(ss_id) ss_id FROM v2_subscription GROUP BY uid, card_id)

而子查询只需要几毫秒。

SELECT MAX(ss_id) ss_id FROM v2_subscription GROUP BY uid, card_id

我在 uid、card_id 和 uid, card_id

上都有索引

这是我的sql，我不知道如何优化它。

请指教，

Answer 1

试试这个，可能会有帮助，如果有帮助请告诉我。

SELECT a.updated, COUNT( * ) count
FROM v2_subscription a 
inner join v2_subscription b 
on a.ss_id = max(b.ss_id)
GROUP BY b.uid, b.card_id

或者这个

SELECT a.updated, COUNT( * ) count
    FROM v2_subscription a 
    inner join v2_subscription b 
    on a.ss_id = (SELECT MAX(b.ss_id) b.ss_id FROM v2_subscription b GROUP BY b.uid, b.card_id)

Answer 2

最后我在@arkumar 上面的答案旁边找到了解决方案。在子查询中添加 "ORDER BY ss_id" 也可以解决问题由于没有order by，子查询的结果没有index.

MySQL 子查询慢

MySQL slow on subquery

mysql

optimization