从Mysql EXPLAIN探寻数据库查询优化(六)
说过， 这样说明Mysql可以找到唯一的行，这个效率是比ref要高的。
再来看一个排序的例子：
mysql> explain select A.id,A.title,B.title from jos_content A left join jos_categories B on A.catid=B.id left join jos_sections C on A.sectionid=C.id order by B.id;
+----+-------------+-------+--------+---------------+---------+---------+-------------------------+-------+---------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+-------------------------+-------+---------------------------------+
| 1 | SIMPLE | A | ALL | NULL | NULL | NULL | NULL | 46585 | Using temporary; Using filesort |
| 1 | SIMPLE | B | eq_ref | PRIMARY | PRIMARY | 4 | joomla_test.A.catid | 1 | |
| 1 | SIMPLE | C | eq_ref | PRIMARY | PRIMARY | 4 | joomla_test.A.sectionid | 1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+-------------------------+-------+---------------------------------+
3 rows in set (0.00 sec)
mysql> explain select A.id,A.title,B.title from jos_content A left join jos_categories B on A.catid=B.id left join jos_sections C on A.sectionid=C.id order by A.id;
+----+-------------+-------+--------+---------------+---------+---------+-------------------------+-------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | www.2cto.com
+----+-------------+-------+--------+---------------+---------+---------+-------------------------+-------+----------------+
| 1 | SIMPLE | A | ALL | NULL | NULL | NULL | NULL | 46585 | Using filesort |
| 1 | SIMPLE | B | eq_ref | PRIMARY | PRIMARY | 4 | joomla_test.A.catid | 1 | |
| 1 | SIMPLE | C | eq_ref | PRIMARY | PRIMARY | 4 | joomla_test.A.sectionid | 1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+-------------------------+-------+----------------+
对于上面两条语句，只是修改了一下排序字段，而第一个使用了Using temporary，而第二个却没有。在日常的网站维护中，如果有Using temporary出现，说明需要做一些优化措施了。
而为什么第一个用了临时表，而第二个没有用呢？
因为如果有ORDER BY子句和一个不同的GROUP BY子句，或者如果ORDER BY或GROUP BY中的字段都来自其他的表而非连接顺序中的第一个表的话，就会创建一个临时表了。
那么，对于上面例子中的第一条语句，我们需要对jos_categories的id进行排序，可以将SQL做如下改动：
mysql> explain select B.id,B.title,A.title from jos_categories A left join jos_content B on A.id=B.catid left join jos_sections C on B.sectionid=C.id order by A.id;
+----+-------------+-------+--------+---------------+-----------+---------+-------------------------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+-----------+---------+-------------------------+------+----------------+
| 1 | SIMPLE | A | ALL | NULL | NULL | NULL | NULL | 18 | Using filesort |
| 1 | SIMPLE | B | ref | idx_catid | idx_catid | 4 | joomla_test.A.id | 3328 | |
| 1 | SIMPLE | C | eq_ref | PRIMARY | PRIMARY | 4 | joomla_test.B.sectionid | 1 | Using index |
+----+-----