get one record at a time from joint table - mysql

Question

I want to get a record from a joint table at a time. But I don't hope the tables are joined as a whole.
The actual tables are as follow.
table contents -- stores content information.
+----+----------+----------+----------+-------------------+
| id | name |status |priority |last_registered_day|
+----+----------+----------+----------+-------------------+
| 1 | content_1|0 |1 |2020/10/10 11:20:20|
| 2 | content_2|2 |1 |2020/10/10 11:21:20|
| 3 | content_3|2 |2 |2020/10/10 11:22:20|
+----+----------+----------+----------+-------------------+
table clusters -- stores cluster information
+----+----------+
| id | name |
+----+----------+
| 1 | cluster_1|
| 2 | cluster_2|
+----+----------+
table content_cluster -- each record indicates that one content is on one cluster
+----------+----------+-------------------+
|content_id|cluster_id| last_update_date|
+----------+----------+-------------------+
| 1 | 1 |2020-10-01T11:30:00|
| 2 | 2 |2020-10-01T11:30:00|
| 3 | 1 |2020-10-01T10:30:00|
| 3 | 2 |2020-10-01T10:30:00|
+----------+----------+-------------------+
By specifying a cluster_id, I want to get one content name at a time where contents.status=2 and (contents name, cluster_id) pair is in content_cluster. The query in sql is something like follow.
SELECT contents.name
FROM contents
JOIN content_cluster
ON contents.content_id = content_cluster.content_id
where contents.status = 2
AND content_cluster.cluster_id = <cluster_id>
ORDER
BY contents.priority
, contents.last_registered_day
, contents.name
LIMIT 1;
However, I don't want the tables to be joined as a whole every time as I have to do it frequently and the tables are large. Is there any efficient way to do this? I can add some indices to the tables. What should I do?

Answer 1

I would try writing the query like this:
SELECT c.name
FROM contents c
WHERE EXISTS (SELECT 1
FROM content_cluster cc
WHERE cc.content_id = c.content_id AND
cc.cluster_id = <cluster_id>
) AND
c.status = 2
ORDER BY c.priority, c.last_registered_day, c.name
LIMIT 1;
Then create the following indexes:
content(status, priority, last_registered_day, name, content_id, name)
content_cluster(content_id, cluster_id).
The goal is for the execution plan to scan the index for context and for each row, look up to see if there is a match in content_cluster. The query stops at the first match.
I can't guarantee that this will generate that plan (avoiding the sort), but it is worth a try.

Answer 2

This query can easily be optimized by applying correct indexes. Apply the alter statements I am mentioning below. And let me know if the performance have considerably increased or not:
alter table contents
add index idx_1 (id),
add index idx_2(status);
alter table content_cluster
add index idx_1 (content_id),
add index idx_2(cluster_id);

Answer 3

If a content can be in multiple clusters and the number of clusters can change, I think that doing a join like this is the best solution.
You could try splitting your contents table into different tables each containing the contents of a specific cluster, but it would need to be updated frequently.