How to make JOIN query use index?

Question

I have two tables:

CREATE TABLE `articles` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(1000) DEFAULT NULL,
  `last_updated` datetime DEFAULT NULL         


        

Answer 1

I would have the following indexes available

articles table -- INDEX ( deleted, last_updated, comment_cnt )

article_categories table -- INDEX ( article_id, category_id ) -- you already have this index

then add Straight_Join to force doing the query as listed instead of it trying to use the article_categories table via whatever statistics it may have to help the query.

SELECT STRAIGHT_JOIN
      a.*
   FROM
      articles AS a
         JOIN article_categories AS c
            ON a.id = c.article_id
            AND c.category_id = 78
   WHERE
          a.deleted = 0
      AND a.comment_cnt > 0
   ORDER BY 
      a.last_updated
   LIMIT 
      100, 20

As per comment / feedback, I would consider reversing based on set if category records is much smaller basis... such as

SELECT STRAIGHT_JOIN
      a.*
   FROM
      article_categories AS c
         JOIN articles as a
            ON c.article_id = a.id
           AND a.deleted = 0
           AND a.Comment_cnt > 0
   WHERE
      c.category_id = 78
   ORDER BY 
      a.last_updated
   LIMIT 
      100, 20

In this case, I would ensure an index on the articles table by

index -- (id, deleted, last_updated)