mysql - 如何优化大表上的计数 SQL 查询

Question

我在 mysql (innodb) 上有一个大表，其中包含产品 Assets (1300 万行)。这是我的数据库的一些模式:

product <-many2one-- file_item --one2many--> family --many2one--> download_type

*file_item* 表是具有数百万行的大表。我尝试使用以下 sql 查询按下载类型计算产品:

select t.name as type, 
count(p.product_id) as n 
from file_item p 
inner join family f on f.id = p.family_id 
inner join type t on f.id_type = t.id 
group by t.id order by t.name;

*file_item* 表有 3 个索引:

• product_family_idx(product_id，family_id)
• family_idx (family_id)
• product_idx (product_id) 解释输出:

+----+-------------+-------+--------+-----------------------------------+---------+---------+-------------------+----------+---------------------------------+
| id | select_type | table | type   | possible_keys                     | key     | key_len | ref               | rows     | Extra                           |
+----+-------------+-------+--------+-----------------------------------+---------+---------+-------------------+----------+---------------------------------+
|  1 | SIMPLE      | p     | ALL    | FAMILY_IDX,PRODUCT_FAMILY_IDX     | NULL    | NULL    | NULL              | 13862870 | Using temporary; Using filesort | 
|  1 | SIMPLE      | f     | eq_ref | PRIMARY,TYPE_ID                   | PRIMARY | 4       | MEDIA.p.FAMILY_IDX|        1 |                                 | 
|  1 | SIMPLE      | t     | eq_ref | PRIMARY                           | PRIMARY | 4       | MEDIA.f.TYPE_ID   |        1 |                                 | 
+----+-------------+-------+--------+-----------------------------------+---------+---------+-------------------+----------+---------------------------------+

查询需要1个多小时才能返回结果。 请问我该如何优化查询？!

Answer 1

这是您的原始查询:

select t.name as type,  
count(p.product_id) as n  
from file_item p  
inner join family f on f.id = p.family_id  
inner join type t on f.id_type = t.id  
group by t.id order by t.name; 

您需要进行两项主要更改:

主要变化#1:重构查询

SELECT A.ProductCount,B.name type
FROM
(
    SELECT id_type id,COUNT(1) ProductCount
    FROM
    (
        SELECT p.id_type
        FROM (SELECT family_id,id_type FROM file_item) p
        INNER JOIN (SELECT id FROM family) f on f.id = p.family_id
    ) AA
    GROUP BY id_type
) A
INNER JOIN type B USING (id)
ORDER BY B.name;

主要变化#2:创建支持重构查询的索引

ALTER TABLE file_item ADD INDEX family_type_idx (family_id,id_type);

试一试!!!