Optimising a working MYSQL statement - mysql

Question

Background
I have a table of "users", a table of "content", and a table of "content_likes". When a user "likes" an item of content, a relation is added to "content_likes". Simple.
Now what I am trying to do is order content based on the number of likes it has received. This is relatively easy, however, I only want to retrieve 10 items at a time and then with a lazy load I am retrieving the next 10 items and so forth. If the select was ordered by time it would be easy to do the offset in the select statement, however, due to the ordering by number of "likes" I need another column I can offset by. So I've added a "rank" column to the result set, then on the next call of 10 items I can offset by this.
This query WORKS and does what I need to do. However, I am concerned about performance. Could anyone advise on optimising this query. Or even possibly a better way of doing it.
DB SCHEMA
CREATE TABLE `users` (
`id` int(11) NOT NULL AUTO_INCREMENT
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8
CREATE TABLE `content` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`owner_id` int(11) NOT NULL,
`added` int(11) NOT NULL,
`deleted` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8
CREATE TABLE `content_likes` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`content_id` int(11) NOT NULL,
`user_id` int(11) NOT NULL,
`added` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8
*columns omitted for simplicity
Breakdown of query
group content_id in content_likes relations table, and order by likes desc
add a column "rank" (or row number) to result set and order by this
join "content" table so that any content with a deleted flag can be ommited
only return results where "rank" (or row number) is greater than variable
limit result set to 10
THE MYSQL
SELECT
results.content_id, results.likes, results.rank
FROM
(
SELECT
t1.content_id, t1.likes, #rn:=#rn+1 AS rank
FROM
(
SELECT
cl.content_id,
COUNT(cl.content_id) AS likes
FROM
content_likes cl
GROUP BY
cl.content_id
ORDER BY
likes DESC,
added DESC
) t1, (SELECT #rn:=0) t2
ORDER BY
rank ASC
) results
LEFT JOIN
content c
ON
(c.id = results.content_id)
WHERE
c.deleted <> 1
AND
results.rank > :lastRank
LIMIT
10
MYSQL ALTERNATIVE
SELECT
*
FROM
(
SELECT
results.*, #rn:=#rn+1 AS rank
FROM
(
SELECT
c.id, cl.likes
FROM
content c
INNER JOIN
(SELECT content_id, COUNT(content_id) AS likes FROM content_likes GROUP BY content_id ORDER BY likes DESC, added DESC) cl
ON
c.id = cl.content_id
WHERE
c.deleted <> 1
AND
c.added > :timeago
LIMIT
100
) results, (SELECT #rn:=0) t2
) final
WHERE
final.rank > :lastRank
LIMIT
5
The "Alternative" mysql query works as I would like it too also. Content is ordered by number of likes by users and I can offset by inserting the last row number. What I have attempted to do here is limit the result sets so if and when the tables get large performance isn't hindered too badly. In this example only content from within a timespan, and limit to 100 will be returned. Then I can offset by the row number (lazy load/pagination)
Any help or advice always appreciated. I am relatively a newbie to mysql so be kind :)

Answer 1

You can eliminate the subquery:
SELECT results.content_id, results.likes, results.rank
FROM (SELECT cl.content_id, COUNT(cl.content_id) AS likes, #rn:=#rn+1 AS rank
FROMc content_likes cl cross join
(SELECT #rn:=0) t2
GROUP BY cl.content_id
ORDER BY likes DESC, added DESC
) results LEFT JOIN
content c
ON c.id = results.content_id
WHERE c.deleted <> 1 AND
results.rank > :lastRank
LIMIT 10;
However, I don't think that will have an appreciable affect on performance. What you should probably do is store the last number number of likes and "added" value and use these to filter the data. The query needs to be a little fixed up, because added is not unambiguously defined in the order by clause:
SELECT results.content_id, results.likes, results.rank, results.added
FROM (SELECT cl.content_id, COUNT(cl.content_id) AS likes, MAX(added) as added, #rn:=#rn+1 AS rank
FROMc content_likes cl cross join
(SELECT #rn := :lastRank) t2
WHERE likes < :likes or
likes = :likes and added < :added
GROUP BY cl.content_id
ORDER BY likes DESC, added DESC
) results LEFT JOIN
content c
ON c.id = results.content_id
WHERE c.deleted <> 1 AND
results.rank > :lastRank
LIMIT 10;
This will at least reduce the number of rows that need to be sorted.