Wrong/Unexpected result with the value optimizer_use_condition_selectivity set to 4

CREATE TABLE t1 (a INT);

INSERT INTO t1 VALUES (1), (2), (3), (4), (5), (6), (7), (8), (9), (10);

set optimizer_use_condition_selectivity=4;

CREATE FUNCTION f1() RETURNS INT DETERMINISTIC

BEGIN

SET @cnt := @cnt + 1;

RETURN 1;

END;|

SET @cnt := 0;

SELECT * FROM t1 WHERE a = f1();

a

1

SELECT @cnt;

@cnt

2

SET @cnt := 0;

set optimizer_use_condition_selectivity=1;

SELECT * FROM t1 WHERE a = f1();

a

1

drop table t1;

drop function f1;