MySQL JOIN with SUBQUERY very slow

Question

I have a forum and I would like to see the latest topics with the author's name and the last user who answered

Table Topic (forum)

| idTopic | IdParent | User | Title | Text             |
--------------------------------------------------------
| 1       | 0        | Max  | Help! | i need somebody  | 
--------------------------------------------------------
| 2       | 1        | Leo  |       | What?!           |  

Query:

SELECT 
    Question.*,
    Response.User AS LastResponseUser 
FROM Topic AS Question
LEFT JOIN (
   SELECT User, IdParent  
   FROM Topic 
   ORDER BY idTopic DESC
) AS Response 
    ON ( Response.IdParent = Question.idTopic )
WHERE Question.IdParent = 0
GROUP BY Question.idTopic
ORDER BY Question.idTopic DESC

Output:

| idTopic | IdParent | User | Title | Text             | LastResponseUser |
---------------------------------------------------------------------------
| 1       | 0        | Max  | Help! | i need somebody  | Leo              |
---------------------------------------------------------------------------

Example: http://sqlfiddle.com/#!2/22f72/4

The query works, but is very slow (more or less 0.90 seconds over 25'000 record).

How can I make it faster?

UPDATE

comparison between the proposed solutions

http://sqlfiddle.com/#!2/94068/22

Answer 1

Assuming the highest IDTopic is the last responses user... and assuming you want to return topics without responses...

Select A.IDTopic, A.IDParent, A.User, A.Title, A.Text, 
case when b.User is null then 'No Response' else B.User end as LastReponseUser
FROM topic A
LEFT JOIN Topic B
 on A.IdTopic = B.IDParent
 and B.IDTopic = (Select max(IDTopic) from Topic 
                  where IDParent=B.IDParent group by IDParent)
WHERE  A.IDParent =0

Answer 2

If using your current schema, I'd recommend adding indexes (particularly a clustered index (primary key)) and simplifying your SQL to let mySQL do the work of optimising the statement, rather than forcing it to run a subquery, sort the results, then run the main query.

CREATE TABLE Topic (
  idTopic  INT
  ,IdParent INT
  ,User     VARCHAR(100)
  ,Title    VARCHAR(255)
  ,Text     VARCHAR(255)       
  ,CONSTRAINT Topic_PK PRIMARY KEY (idTopic) 
  ,CONSTRAINT Topic_idTopic_UK UNIQUE (idTopic) 
  ,INDEX Topic_idParentIdTopic_IX (idParent, idTopic)
);


INSERT INTO Topic (idTopic, IdParent, User, Title, Text) VALUES 
(1, 0, 'Max', 'Help!', 'i need somebody'),
(2, 1, 'Leo', '', 'What!?');


SELECT Question.*
, Response.User AS LastResponseUser 
FROM Topic AS Question
LEFT JOIN Topic AS Response 
  ON Response.IdParent = Question.idTopic
WHERE Question.IdParent = 0
order by Question.idTopic
;

http://sqlfiddle.com/#!2/7f1bc/1

Update

In the comments you mentioned you only want the most recent response. For that, try this:

SELECT Question.*
, Response.User AS LastResponseUser 
FROM Topic AS Question
LEFT JOIN (
  select a.user, a.idParent 
  from Topic as a
  left join Topic as b
  on b.idParent = a.idParent
  and b.idTopic > a.idTopic
  where b.idTopic is null 
) AS Response 
  ON Response.IdParent = Question.idTopic
WHERE Question.IdParent = 0
order by Question.idTopic
;

http://sqlfiddle.com/#!2/7f1bc/3