Question

我有以下查询，这很好，但随着品牌表的增长会变慢：

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)

我不明白为什么它不使用主键作为索引并进行文件排序。如果我用从该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)

为什么会这样？这对我来说真的没有意义。我的意思是，我可以把它分成2个电话，但这看起来很糟糕。我确实注意到我可以通过在子查询中包含一个独特的方式来提高它的效率，但这并没有改变它使用键的方式。

Answer 1

你为什么不写作：

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;

当我包含子查询时，为什么MySQL不使用索引？

1 个答案: