Question

I got 2 tables

First is where Topics are hold

Second is where Messages are hold.

---

I want to make a Recent Topics mod for my project. Its purpose to list most recent 30 topics..

the point is i can easly call

"select * from topics order by date desc limit 30;"

but i dont want to just list recent 30 topics i want to list updating recent 30 topics by messages.

alright let me tell you with a example whats on my mind ! :

lets say there are 5 topics and they are recent.

*Hello Whats UP?
*Good Morning
*Good Afternoon
*Hello Nice!
*There is a Bear behind you!

I wrote a message to “There is a bear behind you” topic. the list must
updated to

*There is a Bear behind you!
*Hello Whats UP?
*Good Morning
*Good Afternoon
*Hello Nice!

I hope you understand my example for what i have to do :/

What i have done so far is this code by searching stackoverflow :

SELECT DISTINCT a.* FROM topics a LEFT JOIN messages b ON a.id = b.topic_id ORDER BY b.date DESC LIMIT 30;

this query works like charm however my topics and messages tables are Freaking HUGE!
this is the explain

+----+-------------+-------+------+---------------+------+---------+-------------------+--------+---------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref               | rows   | Extra                           |
+----+-------------+-------+------+---------------+------+---------+-------------------+--------+---------------------------------+
|  1 | SIMPLE      | a     | ALL  | NULL          | NULL | NULL    | NULL              | 383056 | Using temporary; Using filesort |
|  1 | SIMPLE      | b     | ref  | sira,sira_2   | sira | 4       | avare_sozluk.a.id |      6 | Distinct                        |
+----+-------------+-------+------+---------------+------+---------+-------------------+--------+---------------------------------+

OH! btw i used Distinct for there could be one ore more messages. So query can select the same topic twice or more.. I just want it to select for once to listing..

Anyone can help me with the performance of this query ? or if you have better query for listing like what i told please share :/ i am kinda newbie

Answer 1

A simple workaround would be to add another column to topics table – last_message_at, which will contain date of last message. You will update this field while posting a message.

Now you can do this

SELECT * 
FROM topics 
ORDER BY last_message_at DESC 
LIMIT 30;

Should work pretty fast, if you index last_message_at. Here you trade some write performance for read efficiency. I assume that your users read more than write.