Slow MySQL order by datetime column - mysql

Question

I have news table defined like this:
CREATE TABLE `news` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`creation_date` datetime DEFAULT NULL,
`modification_date` datetime DEFAULT NULL,
`active` bit(1) DEFAULT NULL,
`mark_for_delete` bit(1) DEFAULT NULL,
`verified` bit(1) DEFAULT NULL,
`bot_id` int(11) DEFAULT NULL,
`description` varchar(1000) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL,
`hash` varchar(100) NOT NULL,
`published_at` datetime DEFAULT NULL,
`source` varchar(255) DEFAULT NULL,
`title` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL,
`url` varchar(511) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `UK_1dmji5m90xaiy84vttgkvsub2` (`hash`),
KEY `index_news_source` (`source`),
KEY `index_news_creation_date` (`creation_date`)
) ENGINE=InnoDB AUTO_INCREMENT=30887718 DEFAULT CHARSET=latin1
And a join table to tag news belonging to some popular names:
CREATE TABLE `star_news` (
`stars_id` bigint(20) NOT NULL,
`news_id` bigint(20) NOT NULL,
PRIMARY KEY (`stars_id`,`news_id`),
KEY `FK4eqjn8at6h4d9335q1plxkcnl` (`news_id`),
CONSTRAINT `FK1olc51y8amp8op1kbmx269bac` FOREIGN KEY (`stars_id`) REFERENCES `star` (`id`),
CONSTRAINT `FK4eqjn8at6h4d9335q1plxkcnl` FOREIGN KEY (`news_id`) REFERENCES `news` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
Here is my query to return the latest news
SELECT DISTINCT n.*
FROM news n
JOIN star_news sn
ON n.id = sn.news_id
WHERE sn.stars_id IN (1234, 12345)
ORDER BY n.creation_date DESC
LIMIT 2;
Explain:
+----+-------------+-------+------------+--------+-------------------------------------+---------+---------+-----------------------+------+----------+-----------------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+-------------------------------------+---------+---------+-----------------------+------+----------+-----------------------------------------------------------+
| 1 | SIMPLE | sn | NULL | range | PRIMARY,FK4eqjn8at6h4d9335q1plxkcnl | PRIMARY | 8 | NULL |196225| 100.00 | Using where; Using index; Using temporary; Using filesort |
| 1 | SIMPLE | n | NULL | eq_ref | PRIMARY | PRIMARY | 8 | cosmos_dev.sn.news_id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+-------------------------------------+---------+---------+-----------------------+------+----------+-----------------------------------------------------------+
This query takes 20 seconds on my machine. If I remove the order by clause it returns in sub milli second. How do I make the order by run faster?
I tried using force index on creation_date since its an indexed column, but it worsened the performance

Answer 1

First, write the query as:
SELECT n.*
FROM news n
WHERE EXISTS (SELECT 1
FROM star_news sn
WHERE n.id = sn.news_id AND
sn.stars_id IN (1234, 12345)
)
ORDER BY n.creation_date DESC
LIMIT 2 ;
This eliminates the outer SELECT DISTINCT, which should help.
Then, create an index on star_news(news_id, stars_id). This might also take advantage of an index on news(creation_date desc, id).

Answer 2

So you have 196k news articles relating to those 2 stars? The explain extra tells you what is happening:
Using where; Using index; Using temporary; Using filesort
MySQL is creating a temporary file and sorting it to satisfy the order by because it could not use an index that would facilitate the join AND the ordering of articles by date.