優化MySQL LEFT JOIN查詢

Question

我的目標是選擇primary_category_id （ articles表）或任何輔助類別（ articles_secondary_categories連接表）為給定值的文章。 在此示例查詢中，類別1。我嘗試使用其他類型的聯接，但這里的警告是，文章可能沒有任何二級類別。

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;

歡迎對此優化或替代方法有任何幫助。 在與4K一個DB articles和7K articles_secondary_categories （未分類）需要5秒鍾，運行此查詢。

Answer 1

您可以對次要類別進行反向查詢：

(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;

它應該為您帶來不錯的速度提升-只需確保您為category.articles_id編制索引

Answer 2

避免在where子句中使用OR 。 優化器通常不使用帶有OR謂詞的索引。

嘗試將categories.category_id = 1移到聯接條件中：

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;

該查詢的關鍵字為1 in (ifnull(categories.category_id, primary_category_id), primary_category_id) ，它表示“如果我們加入了類別的連接，請在列表中使用它，否則使用primary_category_id，並且在所有情況下都使用primary_category_id 。