MySQL优化 - 删除某些计数

Question

I'm building a chat for a turn-based game (using PHP and MySQL) where I only want to save the 30 most recent messages in the MySQL database.

Currently, I'm doing 3 queries to delete the oldest message when the count exceeds 30:

1. Insert the new chat message as a new row:

   INSERT INTO chatMessages (userId,matchId,message,timestamp) VALUES (".$params["userId"].",".$params["matchId"].",'".$params["message"]."',NOW())

2. Check if there are too many messages in the database, and get the oldest one's Id.

   SELECT COUNT(id) AS count, id AS oldestId FROM chatMessages WHERE matchId = ".$params["matchId"]." ORDER BY id ASC LIMIT 1

3. Remove the oldest message

   DELETE FROM chatMessages WHERE id = ".$oldestId

Is there any way to do this in 2, or even 1 single query? We have quite a lot of traffic on our servers, so performance is key.

Answer 1

You can reduce the 2nd and 3rd query to one:

delete from chatMessages 
where id not in (select id from chatMessages 
                 order by id desc 
                 limit 30)

You could put that in an insert trigger. That way oldest records would get deleted automatically after an insert.