优化MySQL LEFT JOIN查询

Question

我的目标是选择primary_category_id （ articles表）或任何辅助类别（ articles_secondary_categories连接表）为给定值的文章。 在此示例查询中，类别1。我尝试使用其他类型的联接，但这里的警告是，文章可能没有任何二级类别。

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;

欢迎对此优化或替代方法有任何帮助。 在与4K一个DB articles和7K articles_secondary_categories （未分类）需要5秒钟，运行此查询。

Answer 1

您可以对次要类别进行反向查询：

(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;

它应该为您带来不错的速度提升-只需确保您为category.articles_id编制索引

Answer 2

避免在where子句中使用OR 。 优化器通常不使用带有OR谓词的索引。

尝试将categories.category_id = 1移到联接条件中：

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;

该查询的关键字为1 in (ifnull(categories.category_id, primary_category_id), primary_category_id) ，它表示“如果我们加入了类别的连接，请在列表中使用它，否则使用primary_category_id，并且在所有情况下都使用primary_category_id 。