A hash function controls the address calculation of linear hashing.

linear_hashing.sql

-- http://dev.mysql.com/doc/refman/5.5/en/partitioning-linear-hash.html
DELIMITER $$
DROP FUNCTION IF EXISTS linearHashing$$
CREATE FUNCTION linearHashing(value INT, num INT) RETURNS INT DETERMINISTIC
BEGIN
  SET @v := POWER(2, CEILING(LOG(2, num)));
  SET @n := value & (@v - 1);
  IF @n >= num THEN
    SET @v := CEILING(@v / 2);
    SET @n := @n & (@v - 1);
  END IF;
  RETURN @n;
END;
$$
DELIMITER ;