SET @@sql_mode = '';

CREATE TABLE series (

  val INT(10) UNSIGNED NOT NULL

);

INSERT INTO series VALUES(1);

CREATE TABLE seq_calls (

  c INT

);

INSERT INTO seq_calls VALUES(0);

DELIMITER |;

CREATE FUNCTION next_seq_value() RETURNS INT

BEGIN

 DECLARE next_val INT;

 SELECT val INTO next_val FROM series;

 UPDATE series SET val=mod(val + 1, 2);

 UPDATE seq_calls SET c=c+1;

 RETURN next_val;

END;

|

DELIMITER ;|

CREATE TABLE t1 (t INT, u INT, KEY(t));

INSERT INTO t1 VALUES(10, 10), (11, 11), (12, 12), (12, 13),(14, 15), (15, 16),

                     (16, 17), (17, 18);

ANALYZE TABLE t1;

INSERT INTO t1 SELECT * FROM t1;

ALTER TABLE t1 ADD KEY(t, u);

ANALYZE TABLE t1;

SELECT * FROM series, seq_calls;

SELECT next_seq_value() r, MIN(u) FROM t1 GROUP BY t HAVING r = 1;

DROP TABLE t1;

DROP FUNCTION next_seq_value;

DROP TABLE series, seq_calls;

5.5, 10.0, 10.1  resturn the correct result: