How do you debug MySQL stored procedures?

The following debug_msg procedure can be called to simply output a debug message to the console:

DELIMITER $$

DROP PROCEDURE IF EXISTS `debug_msg`$$
DROP PROCEDURE IF EXISTS `test_procedure`$$

CREATE PROCEDURE debug_msg(enabled INTEGER, msg VARCHAR(255))
BEGIN
  IF enabled THEN
    select concat('** ', msg) AS '** DEBUG:';
  END IF;
END $$

CREATE PROCEDURE test_procedure(arg1 INTEGER, arg2 INTEGER)
BEGIN
  SET @enabled = TRUE;

  call debug_msg(@enabled, 'my first debug message');
  call debug_msg(@enabled, (select concat_ws('','arg1:', arg1)));
  call debug_msg(TRUE, 'This message always shows up');
  call debug_msg(FALSE, 'This message will never show up');
END $$

DELIMITER ;

Then run the test like this:

CALL test_procedure(1,2)

It will result in the following output:

** DEBUG:
** my first debug message
** DEBUG:
** arg1:1
** DEBUG:
** This message always shows up

How to debug a MySQL stored procedure.

Poor mans debugger:

1. Create a table called logtable with two columns, id INT and log VARCHAR(255).

2. Make the id column autoincrement.

3. Use this procedure:

   delimiter //
   DROP PROCEDURE `log_msg`//
   CREATE PROCEDURE `log_msg`(msg VARCHAR(255))
   BEGIN
       insert into logtable select 0, msg;
   END
   

4. Put this code anywhere you want to log a message to the table.

   call log_msg(concat('myvar is: ', myvar, ' and myvar2 is: ', myvar2));
   

It's a nice quick and dirty little logger to figure out what is going on.

Yes, there is a specialized tools for this kind of thing - MySQL Debugger.

I do something very similar to you.

I'll usually include a DEBUG param that defaults to false and I can set to true at run time. Then wrap the debug statements into an "If DEBUG" block.

I also use a logging table with many of my jobs so that I can review processes and timing. My Debug code gets output there as well. I include the calling param name, a brief description, row counts affected (if appropriate), a comments field and a time stamp.

Good debugging tools is one of the sad failings of all SQL platforms.