MYSQL : Improving query perfomance on join with order by clause - mysql

Question

I have two tables which contains the daily activities of a user . I have two join these tables and select top ten ids from this table .
Table 1 : buildlog
+----------------+------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------+------------------------+------+-----+---------+----------------+
| NAME | varchar(50) | YES | | NULL | |
| ID | int(11) | NO | PRI | NULL | auto_increment |
| DATE_AND_TIME | datetime | YES | | NULL | |
| COMMENT | mediumtext | YES | | NULL | |
+----------------+------------------------+------+-----+---------+----------------+
Number Of Rows : 276186
Table 2 : reports
+---------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+----------------+
| r_id | int(10) | NO | PRI | NULL | auto_increment |
| id | int(15) | YES | UNI | NULL | |
| label | varchar(200) | YES | | NULL | |
+---------------+--------------+------+-----+---------+----------------+
Number Of Rows : 134058
If I am using only join query with this two tables using id it comes very quickly .
Query 1:
select buildlog.id,reports.label from buildlog join reports on reports.id = buildlog.id limit 10\G
Query Time : 10 rows in set (0.01 sec)
If I add order by to get latest ten build ids,label it takes 1 to 2 minutes to execute .
Query 2 :
select buildlog.id,reports.label from buildlog join reports on reports.id = buildlog.id order by buildlog.id desc limit 10\G
Query Time : 10 rows in set (0.98 sec)
order by column is an primary key buildlog.id . So, It's already indexed why It takes more time to execute this query ? . Can anyone suggest how can I optimize this?

Answer 1

SELECT * FROM (
SELECT
buildlog.id,
reports.label
FROM
buildlog
JOIN
reports
ON
reports.id = buildlog.id
) AS myval_new
ORDER BY id DESC limit 10
The slow down comes because it is probably choosing to do the ordering before doing the join. Doing the order by in an outer query forces it to only order the selected items.