mysql - 如何使用order by优化mysql group by

Question

我目前在使用 group by 和 order by 时遇到查询速度极慢的情况。我倾向于不使用索引，因为 group by 位于单独的列上，然后 order by

sqlFiddle

Foo 表结构

id -> pk (indexed)
bar_id -> foreign key (indexed)
data -> varchar 
created_at -> timestamp (indexed)

这是查询:

SELECT * FROM foo GROUP BY bar_id ORDER BY created_at DESC

我基本上试图获取每个 bar_id 的最新记录。然而，这最多需要 11 秒才能完成。有没有更好的方法来执行此类查询？

Answer 1

SELECT COUNT(*) FROM foo;
+----------+
| COUNT(*) |
+----------+
|    98304 |
+----------+
1 row in set (0.03 sec)

SELECT x.* 
  FROM foo x
  JOIN
     ( SELECT bar_id
            , MAX(created_at) max_created_at 
         FROM foo
        GROUP
           BY bar_id
     ) y
    ON y.bar_id = x.bar_id 
   AND y.max_created_at = x.created_at;

531 rows in set (0.01 sec)

注意:我稍微修改了您的架构。 http://sqlfiddle.com/#!2/a6296/2