A fast plan with ROR index-merge is ignored when 'index_merge_sort_union=off'

create table t0 (a int);

insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

insert into t0 select a+10 from t0;

insert into t0 select a+20 from t0;

insert into t0 select a+40 from t0;

insert into t0 select a+80 from t0;

insert into t0 select a+160 from t0;

delete from t0 where a > 300;

create table t1 (

  f1 int, f2 int, f3 int, f4 int,

  primary key (f1),  key (f3),  key(f4)

) engine=myisam;

insert into t1 select a+100, a+100, a+100, a+100 from t0;

insert into t1 VALUES (9,0,2,6), (9930,0,0,NULL);

analyze table t1;

set optimizer_switch='index_merge_sort_union=off';

set optimizer_switch='index_merge_union=on';

explain 

select * from t1 

   where (( f3 = 1 or f1 = 7 )  and f1 < 10) or

              (f3 between 2 and 2 and ( f3 = 1 or f4 < 7 ));

insert into t1 values (52,0,1,0),(53,0,1,0);

insert into t1 values (50,0,1,0),(51,0,1,0);

insert into t1 values (48,0,1,0),(49,0,1,0);

insert into t1 values (46,0,1,0),(47,0,1,0);

insert into t1 values (44,0,1,0),(45,0,1,0);

analyze table t1;

explain 

select * from t1 

   where (( f3 = 1 or f1 = 7 )  and f1 < 10) or

              (f3 between 2 and 2 and ( f3 = 1 or f4 < 7 ));

MariaDB [test]> explain 

    -> select * from t1 

    ->    where (( f3 = 1 or f1 = 7 )  and f1 < 10) or

    ->               (f3 between 2 and 2 and ( f3 = 1 or f4 < 7 ));

+------+-------------+-------+-------------+---------------+---------------+---------+------+------+-----------------------------------------+

| id   | select_type | table | type        | possible_keys | key           | key_len | ref  | rows | Extra                                   |

+------+-------------+-------+-------------+---------------+---------------+---------+------+------+-----------------------------------------+

|    1 | SIMPLE      | t1    | index_merge | PRIMARY,f3,f4 | f3,PRIMARY,f3 | 5,4,5   | NULL |    3 | Using union(f3,PRIMARY,f3); Using where |

+------+-------------+-------+-------------+---------------+---------------+---------+------+------+-----------------------------------------+

MariaDB [test]> explain 

    -> select * from t1 

    ->    where (( f3 = 1 or f1 = 7 )  and f1 < 10) or

    ->               (f3 between 2 and 2 and ( f3 = 1 or f4 < 7 ));

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

|    1 | SIMPLE      | t1    | ALL  | PRIMARY,f3,f4 | NULL | NULL    | NULL |  313 | Using where |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+