如果同一表上的多个联接似乎影响性能，如何改进此查询？

Question

以下查询用于在eCommerce网站上搜索产品数据库时应用多个筛选值。

当您使用超过7个过滤器时，MySQL服务器就会崩溃。

• 有机会加入positions_categories_links一次吗？
• 我应该考虑改变数据库结构吗？

每个产品都有一个eshop_cats类别(many2one)和多达20个具有多重关系(many2many)的categories。

表eshop_pos有40,000条记录并包含这些产品。

表eshop_cats有340个记录，包含主要类别。

表categories有6000条记录，包含双工类别。

表positions_categories_links有36万条记录，包含产品和类别之间的密钥。

这是我的疑问：

SELECT COUNT(DISTINCT eshop_pos.id)
FROM eshop_pos
INNER JOIN eshop_cats t1 ON eshop_pos.eshopcatid = t1.id
    AND t1.active = 1
INNER JOIN positions_categories_links t2 ON t2.pos_id = eshop_pos.id
INNER JOIN categories t3 ON t3.id = t2.cat_id
    AND t3.active = 1
    AND t3.section_id = 62021
INNER JOIN positions_categories_links t4 ON t4.pos_id = eshop_pos.id
INNER JOIN categories t5 ON t5.id = t4.cat_id
    AND t5.active = 1
    AND t5.section_id = 62023
INNER JOIN positions_categories_links AS duplex_links_51 ON duplex_links_51.pos_id = eshop_pos.id
    AND duplex_links_51.cat_id = 51
    AND duplex_links_51.value IN (2984)
INNER JOIN positions_categories_links AS duplex_links_52 ON duplex_links_52.pos_id = eshop_pos.id
    AND duplex_links_52.cat_id = 52
    AND duplex_links_52.value IN (3003)
INNER JOIN positions_categories_links AS duplex_links_3904 ON duplex_links_3904.pos_id = eshop_pos.id
    AND duplex_links_3904.cat_id = 3904
    AND duplex_links_3904.value IN (3941)
INNER JOIN positions_categories_links AS duplex_links_4462 ON duplex_links_4462.pos_id = eshop_pos.id
    AND duplex_links_4462.cat_id = 4462
    AND duplex_links_4462.value IN (4465)
INNER JOIN positions_categories_links AS duplex_links_4466 ON duplex_links_4466.pos_id = eshop_pos.id
    AND duplex_links_4466.cat_id = 4466
    AND duplex_links_4466.value IN (4468)
INNER JOIN positions_categories_links AS duplex_links_4472 ON duplex_links_4472.pos_id = eshop_pos.id
    AND duplex_links_4472.cat_id = 4472
    AND duplex_links_4472.value IN (4473)
INNER JOIN positions_categories_links AS duplex_links_4974 ON duplex_links_4974.pos_id = eshop_pos.id
    AND duplex_links_4974.cat_id = 4974
    AND duplex_links_4974.value IN (4978)
INNER JOIN positions_categories_links AS duplex_links_4979 ON duplex_links_4979.pos_id = eshop_pos.id
    AND duplex_links_4979.cat_id = 4979
    AND duplex_links_4979.value IN (4982)
INNER JOIN positions_categories_links AS duplex_links_4984 ON duplex_links_4984.pos_id = eshop_pos.id
    AND duplex_links_4984.cat_id = 4984
    AND duplex_links_4984.value IN (4986)

无法在服务器上运行对查询的解释。然而，它在我的本地笔记本电脑上工作得很好：

​

​

Answer 1

查询的格式略有改变。我还将别名引用从"duplex_links“引用更改为缩写别名"PCL”(来自Position_categories_links表)。简短和相关有助于表引用(至少对我和其他人)。

至于您的表/索引，如果表/索引还不存在，我建议使用下面的表/索引。在本例中，我为您的查询提供了所有覆盖索引，这意味着由于用于满足所有联接条件的列都是索引的一部分，因此sql数据库不必转到底层的实际数据页来确认其他详细信息，从而有助于提高性能。

Table                       Index
eshop_pos                   (id, eshopcatid)
eshop_cats                  (id, active)
positions_categories_links  (pos_id, cat_id, value)
categories                  (id, active, section_id)

我还喜欢显示连接所在位置之间的缩进相关性，以便您知道如何从一个表/别名获取到下一个级别。您可以直接看到事物的层次结构。

SELECT 
      COUNT(DISTINCT eshop_pos.id) 
   FROM 
      eshop_pos 
         inner join eshop_cats t1 
            on eshop_pos.eshopcatid = t1.id 
           AND t1.active = 1 

         inner join positions_categories_links t2 
            on eshop_pos.id = t2.pos_id

            inner join categories t3 
               on t2.cat_id = t3.id
              and t3.active = 1 
              and t3.section_id = 62021 

         inner join positions_categories_links t4 
            on eshop_pos.id = t4.pos_id

            inner join categories t5 
               on t4.cat_id = t5.id
              and t5.active = 1 
              and t5.section_id = 62023 

         INNER JOIN positions_categories_links AS PCL51 
            ON eshop_pos.id = PCL51.pos_id
           AND PCL51.cat_id = 51 
           and PCL51.value in (2984) 

         INNER JOIN positions_categories_links AS PCL52 
            ON eshop_pos.id = PCL52.pos_id
           AND PCL52.cat_id = 52 
           and PCL52.value in (3003) 

         INNER JOIN positions_categories_links AS PCL3904 
            ON eshop_pos.id = PCL3904.pos_id
           AND PCL3904.cat_id = 3904 
           and PCL3904.value in (3941) 

         INNER JOIN positions_categories_links AS PCL4462
            ON eshop_pos.id = PCL4462.pos_id
           AND PCL4462.cat_id = 4462 
           and PCL4462.value in (4465) 

         INNER JOIN positions_categories_links AS PCL4466 
            ON eshop_pos.id = PCL4466.pos_id
           AND PCL4466.cat_id = 4466 
           and PCL4466.value in (4468) 

现在，拥有适当的索引来帮助优化查询是一回事，但是不断地使用这样的多个条件可能会过分。如果您有一个已知的特定细节级别，如特定类别，就像您在这里所做的那样，肯定会有所帮助。

(删除“或”不保证所有标准的版本)