A Little Noise

A stored function to guess which line terminator (‘\r’ or ‘\r\n’) to use with LOAD DATA INFILE

CREATE FUNCTION whichLineTerminator(fileName varchar(255))
RETURNS varchar(20)
NOT DETERMINISTIC
BEGIN
  DECLARE cr_count int;
  DECLARE lf_Count int;
  DECLARE f text;

  SET f := LOAD_FILE(fileName);

  IF f IS NULL THEN RETURN 'Cannot read file.'; END IF;

  SET cr_count := CHARACTER_LENGTH(f) - CHARACTER_LENGTH(REPLACE(f, '\r', ''));
  SET lf_count := CHARACTER_LENGTH(f) - CHARACTER_LENGTH(REPLACE(f, '\n', ''));

  IF cr_count >= lf_count 
  THEN RETURN '\\r\\n';
  ELSE RETURN '\\n';
  END IF;
END

Find the greatest common factor of two positive integers.

CREATE FUNCTION gcd(x int, y int) RETURNS int DETERMINISTIC
BEGIN
  DECLARE dividend int;
  DECLARE divisor int;
  DECLARE remainder int;
  SET dividend := GREATEST(x, y);
  SET remainder := LEAST(x, y);

  WHILE remainder != 0 DO
    SET divisor = remainder;
    SET remainder = MOD(dividend, divisor);
    SET dividend = divisor;
  END WHILE;

  RETURN divisor;
END

Explode a string around a delimiter without a loop.

DROP PROCEDURE IF EXISTS explode;

DELIMITER |

CREATE PROCEDURE explode( pDelim VARCHAR(32), pStr TEXT)                                
BEGIN                                
   DROP TABLE IF EXISTS temp_explode;                                
   CREATE TEMPORARY TABLE temp_explode (id INT AUTO_INCREMENT PRIMARY KEY NOT NULL, word VARCHAR(40));                                
   SET @sql := CONCAT('INSERT INTO temp_explode (word) VALUES (', REPLACE(QUOTE(pStr), pDelim, '\'), (\''), ')');                                
   PREPARE myStmt FROM @sql;                                
   EXECUTE myStmt;                                
END |   

DELIMITER ;

-- example call:
SET @str  = "The quick brown fox jumped over the lazy dog"; 
SET @delim = " "; 

CALL explode(@delim,@str);
SELECT id,word FROM temp_explode;

Extract the first, longest pattern that matches a regular expression.
You could add a parameter or write a similar function to get the shortest match instead (by starting with e = s and then incrementing e, instead of starting with e = LENGTH(string) and decrementing).

CREATE FUNCTION REGEXP_EXTRACT(string TEXT, exp TEXT)
-- Extract the first longest string that matches the regular expression
-- If the string is 'ABCD', check all strings and see what matches: 'ABCD', 'ABC', 'AB', 'A', 'BCD', 'BC', 'B', 'CD', 'C', 'D'
-- It's not smart enough to handle things like (A)|(BCD) correctly in that it will return the whole string, not just the matching token.

RETURNS TEXT
DETERMINISTIC
BEGIN
  DECLARE s INT DEFAULT 1;
  DECLARE e INT;
  DECLARE adjustStart TINYINT DEFAULT 1;
  DECLARE adjustEnd TINYINT DEFAULT 1;

  -- Because REGEXP matches anywhere in the string, and we only want the part that matches, adjust the expression to add '^' and '$'
  -- Of course, if those are already there, don't add them, but change the method of extraction accordingly.

  IF LEFT(exp, 1) = '^' THEN 
    SET adjustStart = 0;
  ELSE
    SET exp = CONCAT('^', exp);
  END IF;

  IF RIGHT(exp, 1) = '$' THEN
    SET adjustEnd = 0;
  ELSE
    SET exp = CONCAT(exp, '$');
  END IF;

  -- Loop through the string, moving the end pointer back towards the start pointer, then advance the start pointer and repeat
  -- Bail out of the loops early if the original expression started with '^' or ended with '$', since that means the pointers can't move
  WHILE (s <= LENGTH(string)) DO
    SET e = LENGTH(string);
    WHILE (e >= 1) DO
      IF SUBSTRING(string, s, e) REGEXP exp THEN
        RETURN SUBSTRING(string, s, e);
      END IF;
      IF adjustEnd THEN
        SET e = e - 1;
      ELSE
        SET e = s - 1; -- ugh, such a hack to end it early
      END IF;
    END WHILE;
    IF adjustStart THEN
      SET s = s + 1;
    ELSE
      SET s = LENGTH(string) + 1; -- ugh, such a hack to end it early
    END IF;
  END WHILE;

  RETURN NULL;

END

Implement multi-master to single-slave replication by periodically saving state and changing masters, and do it all in SQL.*

Just a proof of concept. I suggest you add some error checking (like, don’t make the switch if the slave isn’t running).

Remember that all this stuff goes on the SLAVE. If you absent-mindedly log in to the master, like I did a few times, you might wonder why your relay_log.info file isn’t read.

Set up a table to keep track of each master file. Mine use the same user name and password for all hosts; you can add those fields and adjust the procedure accordingly if you like.

USE mysql;
DROP TABLE IF EXISTS rotate_master;
CREATE TABLE rotate_master (
id int auto_increment primary key,
master_host varchar(255),
master_port int unsigned,
master_log_file varchar(255),
master_log_pos int unsigned,
in_use boolean DEFAULT 0
);

Insert the list of masters. The one that is currently master should have `in_use` set to 1; all the others should be 0. The order you insert them is the order they will rotate.

INSERT INTO rotate_master VALUES (NULL, 'localhost', 3306, 'bin.000001', 0, 1);
INSERT INTO rotate_master VALUES (NULL, 'localhost', 3308, 'bin.000001', 0, 0);

Then, a procedure to grab the current position (by reading from the relay_log.info file, since the slave position is not otherwise exposed in any accessible manner), record it in the above table, and change masters to the next one in the list.

DROP PROCEDURE IF EXISTS rotate_master;
DELIMITER //

-- fetch next host
SELECT
id,
master_host,
master_port,
master_log_file,
master_log_pos
INTO _id, _master_host, _master_port, _master_file, _master_pos
FROM rotate_master
ORDER BY id <= LAST_INSERT_ID(), id LIMIT 1; -- advance to next host SET @sql := CONCAT( 'CHANGE MASTER TO master_host=', QUOTE(_master_host), ', master_port=', _master_port, ', master_log_file=', QUOTE(_master_file), ', master_log_pos=', _master_pos); PREPARE myStmt FROM @sql; EXECUTE myStmt; -- mark host as changed UPDATE mysql.rotate_master SET in_use = 0 WHERE in_use = 1; UPDATE mysql.rotate_master SET in_use = 1 WHERE id = _id; START SLAVE; END// DELIMITER ;

Finally, schedule it to rotate. I used 10 seconds just to play with; I imagine a few minutes would be more practical. *If you don't have 5.1 or later, you'll have to schedule this part externally, with cron or task scheduler.

DROP EVENT IF EXISTS rotate_master;
CREATE EVENT rotate_master
ON SCHEDULE EVERY 10 SECOND
DO CALL mysql.rotate_master();

csplit backup.sql /"^-- Current Database"/ {*}; for f in xx*; do cat xx00 "$f" | mysql& done;

and then rm xx* when the background jobs have finished.

I haven’t done any benchmarking, and I haven’t worked out what to do if the number of databases exceeds the number of available connections.

So you have two versions of a database sitting around – maybe you’ve been trying things out on your development box, and now you want to move it all over to production. If it’s just a new table here and an extra column there, you could do it by hand, but if you’ve made lots of changes, it would be nice to let the computer do it for you.

Enter MySQL Workbenchmysqldiff.