DELIMITER $$
CREATE FUNCTION f_format_text(str VARCHAR (255), replacement VARCHAR(1), ply TINYINT(3), bias DECIMAL(5,2)) RETURNS VARCHAR(255)
  BEGIN
    DECLARE treshold INT;
    DECLARE len INT;
    SET len = LENGTH(str);
    SET treshold = FLOOR(ply*bias*len);

    RETURN (
SELECT REPLACE(str, SUBSTRING(str,treshold), REPEAT(replacement, ((len - treshold) + 1)))
    );
  END $$
DELIMITER ;