Order by with distinct has an impact on performance - mysql

Question

I have .6 million data set. Now I am trying to sort the data set by timestamp, and also due to one to many relationship I have to do some Inner JOIN and use distinct.
My Query is like below,
SELECT DISTINCT p.id, s.subject, p.joining_time
FROM profile p
INNER JOIN profile_subject ps ON p.id=ps.profile_id
LEFT JOIN subject s ON ps.subject_id=s.id
ORDER BY p.joining_time LIMIT 20;
Now this query is taking almost 28 sec
But without order by clause its taking only 0.11 sec
How to improve this query with desired result?

Answer 1

My simplest suggestion is to put an index on profile(joining_time). Then select a certain number of the most recent in a subquery. For instance, if you are pretty confident that the top 20 rows you want are within the most recent 100 records in profile, then you can try this:
SELECT DISTINCT p.id, s.subject, p.joining_time
FROM (SELECT p.id, p.joining_join
FROM profile p
ORDER BY p.joining_time
LIMIT 100
) p INNER JOIN
profile_subject ps
ON p.id = ps.profile_id LEFT JOIN
subject s
ON ps.subject_id = s.id
ORDER BY p.joining_time
LIMIT 20;
I would also suggest that you remove the DISTINCT keyword. Unless you have duplicate subjects for one profile, then this is not necessary. Similarly, it is hard to believe that the LEFT JOIN is necessary. In a well-structured database, there would be no subject_id values in profile_subject that are not in subject. So, try this:
SELECT p.id, s.subject, p.joining_time
FROM (SELECT p.id, p.joining_join
FROM profile p
ORDER BY p.joining_time
LIMIT 100
) p INNER JOIN
profile_subject ps
ON p.id = ps.profile_id JOIN
subject s
ON ps.subject_id = s.id
ORDER BY p.joining_time
LIMIT 20;