distinct和order by共存对索引选择的影响
程序需要找出产品表中最新添加的状态为可用的20个产品用于客户展示sql语句如下
SELECT DISTINCT p.products_id, p.sj_id, p.px_id, p.products_model, p.products_p_w_picpath, pd.products_name
FROM products AS p LEFT JOIN products_description AS pd ON pd.products_id=p.products_id
WHERE 1 AND p.products_status=1 ORDER BY p.products_date_added DESC LIMIT 0, 20; 功能上线后,发现非常慢。
explian
+----+-------------+-------+------+---------------+---------+---------+-----------------------------+-------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows| Extra |
+----+-------------+-------+------+---------------+---------+---------+-----------------------------+-------+----------------------------------------------+
|1 | SIMPLE | p | ALL| NULL | NULL | NULL | NULL | 71180 | Using where; Using temporary; Using filesort |
|1 | SIMPLE | pd | ref| PRIMARY | PRIMARY | 4 | banggood_work.p.products_id | 1 | |
+----+-------------+-------+------+---------------+---------+---------+-----------------------------+-------+----------------------------------------------+ 发现products_date_added索引并没有被用上(products_date_added字段有索引,99.9%以上为products_status=1的产品),应该来讲，mysql优化器此时会选择products_date_added字段上的索引.
仔细观察sql发现有distinct关键字,而这语句根本不需要distinct.
此时mysql会将整个products取出来,进行去重,导致了全表扫！
将distinct去掉，再explain
+----+-------------+-------+-------+---------------+---------------------+---------+-----------------------------+------+-------------+
| id | select_type | table | type| possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------------------+---------+-----------------------------+------+-------------+
|1 | SIMPLE | p | index | NULL | products_date_added | 8 | NULL | 20 | Using where |
|1 | SIMPLE | pd | ref | PRIMARY | PRIMARY | 4 | banggood_work.p.products_id | 1 | |
+----+-------------+-------+-------+---------------+---------------------+---------+-----------------------------+------+-------------+ 发现mysql已经智能的选择了products_date_added索引，尽管该字段不再where条件上。
总结
1. mysql选择索引并不一定只会在join条件和where条件上选择,有时候也会智能的选择order by、group by上的字段
2.distinct会干扰order by智能选择索引。distinct和order by共存对索引是有影响的，一般是在非where条件选择索引的情况下
3. 请慎重使用distinct，不需要尽量不要用！另外可以参考我的另外一篇博文http://qdjalone.blog.51cto.com/1222376/1360112
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