Waarom veroorzaakt deze MySQL-trigger een stack-overflow?

Ik kwam vandaag hetzelfde probleem tegen, waarbij elke trigger een stack-overrun veroorzaakte. Bleek dat mijn Zend Community Server-installatie wordt geleverd met een standaard my.cnf-bestand waarin de thread_stack-grootte was ingesteld op 128K, wat resulteerde in 131072 bytes beschikbaar voor de stapel in elke thread:

mysql> show variables where `Variable_name` = 'thread_stack';
+---------------+--------+
| Variable_name | Value  |
+---------------+--------+
| thread_stack  | 131072 |
+---------------+--------+

Dus ik becommentarieerde de regel in /usr/local/zend/mysql/data/my.cnf, herstartte de mysql-daemon en voila! De standaard 192K is

mysql> show variables where `Variable_name` = 'thread_stack';
+---------------+--------+
| Variable_name | Value  |
+---------------+--------+
| thread_stack  | 196608 |
+---------------+--------+

Nu werken je table &tchester's trigger perfect :) (let wel op het scheidingsteken)

mysql> CREATE TABLE `job_title` (
    ->   `job_id` int(11) NOT NULL AUTO_INCREMENT,
    ->   `position_id` int(11) DEFAULT NULL,
    ->   `title` varchar(255) COLLATE latin1_general_cs NOT NULL,
    ->   `selectable` tinyint(4) NOT NULL DEFAULT '0',
    ->   PRIMARY KEY (`job_id`),
    ->   UNIQUE KEY `title` (`title`)
    -> ) ENGINE=InnoDB;
Query OK, 0 rows affected (0.14 sec)

mysql> DELIMITER &&
mysql> create trigger job_position_trigger   
    ->   before insert on job_title for each row  
    -> begin    
    ->     if new.position_id is null then       
    ->        set @position = (select max(position_id)+1 from job_title);
    ->        if @position is null then set @position = 1; end if;
    ->        set new.position_id = @position;    
    ->     end if;  
    -> end; 
    -> &&
Query OK, 0 rows affected (0.29 sec)

mysql> DELIMITER ;
mysql> insert into job_title (title, selectable) values ("test", 1);
Query OK, 1 row affected (0.00 sec)

mysql> insert into job_title (title, selectable) values ("test2", 3);
Query OK, 1 row affected (0.00 sec)

mysql> select * from job_title;
+--------+-------------+-------+------------+
| job_id | position_id | title | selectable |
+--------+-------------+-------+------------+
|      1 |           1 | test  |          1 |
|      2 |           2 | test2 |          3 |
+--------+-------------+-------+------------+
2 rows in set (0.00 sec)

De fout die je kreeg, 9024 bytes gebruikt van een stapel van 131072 bytes en 128000 bytes nodig, is logisch:9024 + 128000> 131072.