MySQL Stored Procedure Programming

Name

LOOP-01: Make sure the loop will terminate

Synopsis

One of the most annoying and potentially disruptive bugs that can be created in any language is the inadvertent infinite loop.

Making sure that a loop will terminate requires that you simulate all possible paths through the loop and assure yourself that the loop will always encounter an exit condition. If the loop does not terminate, it will likely consume excessive CPU and/or memory resources until it is manually terminated by the system administrator. In a worst-case scenario, the MySQL server itself may be terminated.

Example

The following stored procedure calculates the number of prime numbers less than the supplied input parameter. It's part of a larger routine that we plan to put in action when we're next contacted by extraterrestrial intelligences that announce their presence by broadcasting prime numbers at planet Earth.

 CREATE PROCEDURE check_for_primes(in_limit INT) BEGIN DECLARE i INT DEFAULT 2; DECLARE j INT DEFAULT 1; DECLARE n_primes INT DEFAULT 0; DECLARE is_prime INT DEFAULT 0; REPEAT -- See if i is a prime number SET j=2; SET is_prime=1; divisors: WHILE(j< i) DO IF MOD(i,j)=0 THEN SET is_prime=0; LEAVE divisors; END IF; SET j=j+1; END WHILE; IF is_prime THEN SET n_primes=n_primes+1; END IF; -- Move onto the next number IF (MOD(i,2)=0) THEN SET i=i+1; ELSE -- Next number is even, no need -- to check for it as a prime SET i=i+2; END IF; UNTIL (i=in_limit) END REPEAT; SELECT CONCAT(n_primes,' prime numbers <= ',in_limit); ...