Question

I am using the trigger shown below to automatically assign a random key to inserted rows, retrying the random generation if an existing row is found. 99.9% of the time this works exactly as expected. However, every couple of weeks or so the trigger locks up, the REPEAT loop within it apparently becoming infinite; the query continues indefinitely, effectively locking up the application, and has to be KILLed.

As far as I can see, there is nothing about the logic involved that makes it seem as if this should ever happen. The keyspace involved is certainly nowhere near exhaustion and never will be. What is going on?

The trigger:

DELIMITER //;
CREATE TRIGGER `mytable_insert` BEFORE INSERT ON `mytable`
FOR EACH ROW
BEGIN
    DECLARE `curr` BIGINT UNSIGNED;
    DECLARE `sel` BIGINT UNSIGNED;
    IF NEW.`mykey` IS NULL THEN
        REPEAT
            SELECT FLOOR(RAND() * 18446744073709551615) INTO `sel`;
            SELECT `id` INTO `curr` FROM `mytable` WHERE `mykey` = `sel` LIMIT 1;
        UNTIL `curr` IS NULL END REPEAT;
        SET NEW.`mykey` = `sel`;
    END IF;
END

Answer 1

The error is this line

SELECT `id` INTO `curr` FROM `mytable` WHERE `mykey` = `sel` LIMIT 1;

If no row matches mykey = sel then curr isn’t assigned at all.
So curr will never be null if it’s ever assigned anything else.

Replace with something like this (untested)

DELIMITER //;
CREATE TRIGGER `mytable_insert` BEFORE INSERT ON `mytable`
FOR EACH ROW
BEGIN
    DECLARE `count` INT UNSIGNED;
    DECLARE `sel` BIGINT UNSIGNED;
    IF NEW.`mykey` IS NULL THEN
        REPEAT
            SELECT FLOOR(RAND() * 18446744073709551615) INTO `sel`;
            SELECT count(1) INTO `count` FROM `mytable` WHERE `mykey` = `sel`;
        UNTIL `count` = 0 END REPEAT;
        SET NEW.`mykey` = `sel`;
    END IF;
END