为什么MySQL在我包含子查询时不使用索引？

Question

我有以下查询，这是很好的，但将获得更慢的品牌表的增长：

mysql> explain select brand_id as id,brands.name from tags 
    ->      INNER JOIN brands on tags.brand_id = brands.id 
    ->      where brand_id in 
    ->      (select brand_id from tags where outfit_id in 
    ->      (1,6,68,265,271)) 
    ->      group by brand_id, brands.name 
    ->      ORDER BY count(brand_id) 
    ->      LIMIT 5; 
+----+--------------------+--------+----------------+------------------------------------------------+------------------------+---------+-----------------+------+----------------------------------------------+ 
| id | select_type  | table | type   | possible_keys         | key     | key_len | ref    | rows | Extra          | 
+----+--------------------+--------+----------------+------------------------------------------------+------------------------+---------+-----------------+------+----------------------------------------------+ 
| 1 | PRIMARY   | brands | ALL   | PRIMARY          | NULL     | NULL | NULL   | 165 | Using where; Using temporary; Using filesort | 
| 1 | PRIMARY   | tags | ref   | index_tags_on_brand_id       | index_tags_on_brand_id | 5  | waywn.brands.id | 1 | Using where; Using index      | 
| 2 | DEPENDENT SUBQUERY | tags | index_subquery | index_tags_on_outfit_id,index_tags_on_brand_id | index_tags_on_brand_id | 5  | func   | 1 | Using where         | 
+----+--------------------+--------+----------------+------------------------------------------------+------------------------+---------+-----------------+------+----------------------------------------------+ 
3 rows in set (0.00 sec) 

为什么不使用主键索引这里做了，我不明白文件排序。如果我用从子查询返回的值替换子查询，MySQL正确使用索引：

mysql> explain select brand_id as id,brands.name from tags 
    ->      INNER JOIN brands on tags.brand_id = brands.id 
    ->      where brand_id in 
    ->      (2, 2, 9, 10, 40, 32, 9, 118) 
    ->      group by brand_id, brands.name 
    ->      ORDER BY count(brand_id) 
    ->      LIMIT 5; 
+----+-------------+--------+-------+------------------------+------------------------+---------+-----------------+------+----------------------------------------------+ 
| id | select_type | table | type | possible_keys   | key     | key_len | ref    | rows | Extra          | 
+----+-------------+--------+-------+------------------------+------------------------+---------+-----------------+------+----------------------------------------------+ 
| 1 | SIMPLE  | brands | range | PRIMARY    | PRIMARY    | 4  | NULL   | 6 | Using where; Using temporary; Using filesort | 
| 1 | SIMPLE  | tags | ref | index_tags_on_brand_id | index_tags_on_brand_id | 5  | waywn.brands.id | 1 | Using where; Using index      | 
+----+-------------+--------+-------+------------------------+------------------------+---------+-----------------+------+----------------------------------------------+ 
2 rows in set (0.00 sec) 

mysql> explain select brand_id from tags where outfit_id in      (1,6,68,265,271); 
+----+-------------+-------+-------+-------------------------+-------------------------+---------+------+------+-------------+ 
| id | select_type | table | type | possible_keys   | key      | key_len | ref | rows | Extra  | 
+----+-------------+-------+-------+-------------------------+-------------------------+---------+------+------+-------------+ 
| 1 | SIMPLE  | tags | range | index_tags_on_outfit_id | index_tags_on_outfit_id | 5  | NULL | 8 | Using where | 
+----+-------------+-------+-------+-------------------------+-------------------------+---------+------+------+-------------+ 
1 row in set (0.00 sec) 

这是为什么？这对我来说并不合适。我的意思是，我可以把它分成2个电话，但这似乎很差。我确实注意到，通过在子查询中包含独特的内容，我可以使其效率稍高一些，但这并没有改变它使用密钥的方式。

Answer 1

你为什么不写探微：

SELECT brand_id as id,brands.name 
FROM tags 
INNER JOIN brands ON tags.brand_id = brands.id 
WHERE outfit_id in (1,6,68,265,271) 
GROUP BY brand_id, brands.name 
ORDER BY count(brand_id) 
LIMIT 5;