MySQL Error when trigger calls a procedure - mysql

Question

I have a table (ft_ttd) and want to sort it descending (num) and insert rating numbers into rating column.
Initial Table http://dl.dropbox.com/u/3922390/2.png
Something like that:
Result Table http://dl.dropbox.com/u/3922390/1.png
I've created a procedure.
CREATE PROCEDURE proc_ft_ttd_sort
BEGIN
CREATE TEMPORARY TABLE ft_ttd_sort
(id int (2),
num int (3),
rating int (2) AUTO_INCREMENT PRIMARY KEY);
INSERT INTO ft_ttd_sort (id, num) SELECT id, num FROM ft_ttd ORDER BY num DESC;
TRUNCATE TABLE ft_ttd;
INSERT INTO ft_ttd SELECT * FROM ft_ttd_sort;
DROP TABLE ft_ttd_sort;
END;
When I call it - it works great.
CALL proc_ft_ttd_sort;
After that I've created trigger calling this procedure.
CREATE TRIGGER au_ft_ttd_fer AFTER UPDATE ON ft_ttd FOR EACH ROW
BEGIN
CALL proc_ft_ttd_sort();
END;
Now every time when I update ft_ttd table I've got a error.
UPDATE ft_ttd SET num = 9 WHERE id = 3;
ERROR 1422 (HY000): Explicit or implicit commit is not allowed in stored function ortrigger.
Any ideas how to make it work? Maybe this process can be optimized?
Thank you!

Answer 1

The create table statement is an implicit commit, since it's DDL. Basically, the answer is you can't create a table in a trigger.
http://dev.mysql.com/doc/refman/5.0/en/stored-program-restrictions.html

Answer 2

Triggers can't do it
DDL aside, your trigger-based approach has a few difficulties. First, you want to modify the very table that's been updated, and that's not permitted in MySQL 5.
Second, you really want a statement-level trigger rather than FOR EACH ROW — no need to re-rank the whole table for every affected row — but that's not supported in MySQL 5.
Dynamically compute "rating"
So ... is it enough to just compute rating dynamically using a MySQL ROW_NUMBER() workaround?
-- ALTER TABLE ft_ttd DROP COLUMN rating; -- if you like
SELECT id,
num,
#i := #i + 1 AS rating
FROM ft_ttd
CROSS JOIN (SELECT #i := 0 AS zero) d
ORDER BY num DESC;
Unfortunately, you cannot wrap that SELECT in a VIEW (since a view's "SELECT statement cannot refer to system or user variables"). However, you could hide that in a selectable stored procedure:
CREATE PROCEDURE sp_ranked_ft_ttd () BEGIN
SELECT id, num, #i := #i + 1 AS rating
FROM ft_ttd CROSS JOIN (SELECT #i := 0 AS zero) d
ORDER BY num DESC
END
Or UPDATE if you must
As a kluge, if you must store rating in the table rather than compute it, you can run this UPDATE as needed:
UPDATE t
CROSS JOIN ( SELECT id, #i := #i + 1 AS new_rating
FROM ft_ttd
CROSS JOIN (SELECT #i := 0 AS zero) d
ORDER BY num DESC
) ranked
ON ft_ttd.id = ranked.id SET ft_ttd.rating = ranked.new_rating;
Now instruct your client code to ignore rows where rating IS NULL — those haven't been ranked yet. Better, create a VIEW that does that for you.
Kluging further, you can likely regularly UPDATE via CREATE EVENT.