不要随随便便的distinct和order by

POOPE

　　有客户反应网站后台订单相关查询非常慢,通过程序拿到了相关sql
　　explain

explain SELECT DISTINCT(o.orders_id), o.oa_order_id, customers_email_address, o.order_type, ot.text AS total_value, o.track_number, o.date_purchased, o.orders_status, o.specialOperate, o.isSpecialParent, o.pay_ip, o.supply_id, o.products_center_id, o.split_code, o.is_import, o.shipDays,o.delivery_country,o.use_coupon ,o.payment_method FROM orders AS o LEFT JOIN orders_total AS ot ON ot.orders_id=o.orders_id AND ot.class='ot_total' WHERE 1  AND o.is_delete = 0  AND o.date_purchased >= '2013-09-30 10:00:00' AND (o.specialOperate = 0 OR o.isSpecialParent=1) ORDER BY date_purchased DESC, orders_id DESC LIMIT 0, 20;
+----+-------------+-------+-------+----------------------------------+----------------------------+---------+----------------------+--------+----------------------------------------------+
| id | select_type | table | type  | possible_keys          | key            | key_len | ref          | rows   | Extra                    |
+----+-------------+-------+-------+----------------------------------+----------------------------+---------+----------------------+--------+----------------------------------------------+
|  1 | SIMPLE    | o   | range | date_purchased           | date_purchased       | 9     | NULL         | 606632 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE    | ot  | ref   | idx_orders_total_orders_id,class | idx_orders_total_orders_id | 4     | banggood.o.orders_id |   19 |                        |
+----+-------------+-------+-------+----------------------------------+----------------------------+---------+----------------------+--------+----------------------------------------------+
2 rows in set (0.05 sec)

　　发现索引使用正常,执行状态中发现有Copying to tmp table on disk状态，执行时间超过50s。
　　使用profiling发现Copying to tmp table on disk占用了大部分性能。
　　仔细查看该语句并和开发讨论，发现distinct和ORDER BY date_purchased DESC, orders_id DESC中，distinct关键字可以省略,而且ORDER BY date_purchased DESC, orders_id DESC可以去掉后面的orders_id desc(开发对多个字段排序不理解).
　　去掉后，再次explain

mysql> EXPLAIN
  -> SELECT o.orders_id, o.oa_order_id, customers_email_address, o.order_type, ot.text AS total_value, o.track_number, o.date_purchased, o.orders_status, o.specialOperate, o.isSpecialParent, o.pay_ip, o.supply_id, o.products_center_id, o.split_code, o.is_import, o.shipDays,o.delivery_country,o.use_coupon ,o.payment_method FROM orders AS o LEFT JOIN orders_total AS ot ON ot.orders_id=o.orders_id AND ot.class='ot_total' WHERE 1  AND o.is_delete = 0  AND o.date_purchased >= '2013-09-30 10:00:00' AND (o.specialOperate = 0 OR o.isSpecialParent=1)
  -> ORDER BY date_purchased DESC LIMIT 0, 20;
+----+-------------+-------+-------+----------------------------------+----------------------------+---------+----------------------+--------+-------------+
| id | select_type | table | type  | possible_keys          | key            | key_len | ref          | rows   | Extra     |
+----+-------------+-------+-------+----------------------------------+----------------------------+---------+----------------------+--------+-------------+
|  1 | SIMPLE    | o   | range | date_purchased           | date_purchased       | 9     | NULL         | 606632 | Using where |
|  1 | SIMPLE    | ot  | ref   | idx_orders_total_orders_id,class | idx_orders_total_orders_id | 4     | banggood.o.orders_id |   19 |       |
+----+-------------+-------+-------+----------------------------------+----------------------------+---------+----------------------+--------+-------------+
2 rows in set (0.01 sec)

　　索引使用情况不变,但是下面的profiling,发现结果瞬间出来,执行时间不过0.003s,而且已经没有了Copying to tmp table on disk状态。
　　

　　

　　总结：1.因为distinct关键字需要对结果集进行去重,如果天然无重复,是不需要加上去重关键字的,上面的例子结果集有将近百万,去重字段又多，在tmp_table_size以及sort_buffer_size中排序已经不够用,所以将结果集复制到磁盘，严重影响速度
　　2. order by a,b 开发人员很喜欢用类似的语句，尽管对功能没有多大作用