优化MySQL左连接查询

Question

我的目标是选择primary_category_id (articles表)或任何二级类别(articles_secondary_categories连接表)是给定值的文章。在这个示例查询中，类别1。我尝试使用其他类型的连接，但这里需要注意的是，一篇文章可能没有任何辅助类别。

SELECT DISTINCT articles.* 
FROM articles
LEFT JOIN articles_secondary_categories AS categories 
    ON categories.article_id = articles.id
WHERE 
(
    primary_category_id = 1
    OR
    categories.category_id = 1
)
    AND articles.state = "published"
    AND edition_id = 1

ORDER BY publish_at DESC
LIMIT 10;

欢迎任何优化这一点或替代方案的帮助。在具有4k articles和7k articles_secondary_categories (非类别)的数据库中，运行此查询需要5秒。

Answer 1

您可以反转对辅助类别的查询：

(SELECT articles.* 
FROM articles
WHERE primary_category_id = 1)
UNION DISTINCT
(SELECT articles.*
FROM articles_secondary_categories AS categories 
JOIN articles ON (categories.article_id = articles.id)
WHERE categories.category_id = 1
GROUP BY articles_id)
ORDER BY publish_at DESC
LIMIT 10;

它应该会给你一个不错的速度提升-只要确保你索引了categories.articles_id

Answer 2

避免在where子句中使用OR。优化器通常不会将索引与OR谓词一起使用。

尝试将categories.category_id = 1移动到联接条件中：

SELECT articles.* 
FROM articles
LEFT JOIN articles_secondary_categories AS categories 
    ON categories.article_id = articles.id and categories.category_id = 1
WHERE 1 in (ifnull(categories.category_id, primary_category_id), primary_category_id)
AND articles.state = "published"
AND edition_id = 1
ORDER BY publish_at DESC
LIMIT 10;

这个查询的关键是1 in (ifnull(categories.category_id, primary_category_id), primary_category_id)，它说“如果我们连接到类别，就在列表中使用它，否则使用primary_category_id，并且在所有情况下都使用primary_category_id。