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

Use of a non-covering index is expensive. For each row, any uncovered columns have to be retrieved from the base table, using the primary key. So I'd first try to make the index on articles covering. That might help convince the MySQL query optimizer that the index is useful. For example:

KEY IX_Articles_last_updated (last_updated, id, title, comment_cnt, deleted),

If that doesn't help, you could play around with FORCE INDEX:

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

The name of the index enforcing the primary key is always "primary".