为此简单查询正确的MySQL索引

Question

I have a simple query that does a group by and order by (with a join), but it doesn't seem to be working without temporary tables, and filesort. Here is the query:

SELECT items.name, max(activity.created_at), count(items.item_id) as total_count
FROM activity
INNER JOIN items ON activity.item_id = items.item_id 
WHERE activity.user_id = 'XXX'  
GROUP BY activity.item_id 
order by activity.created_at desc
LIMIT 0, 15

I have an index on:

(activity.item_id, activity.user_id, activity.created_at) - idx_item_created 
(activity.user_id, activity.created_at) - user_id_2

When I force idx_item_created it does a full table scan, by default it runs using user_id_2 and always produces:

Using where; Using temporary; Using filesort

Anyway to optimize this query better?

Answer 1

You are going to have to have a filesort on this, because you have group by and order by with two different expressions. For this query:

SELECT items.name, max(activity.created_at), count(items.item_id) as total_count
FROM activity INNER JOIN
     items
     ON activity.item_id = items.item_id 
WHERE activity.user_id = 'XXX'  
GROUP BY activity.item_id 
order by activity.created_at desc
LIMIT 0, 15

The best index is a covering index on activity : activity(user_id, item_id, created_at) . This will not eliminate the file sort, though.