The following query:

SELECT * FROM t1 FORCE KEY (PRIMARY,f3,f4)
WHERE f2
AND ( f3 IS NULL OR f1 = 7 )
AND f1 NOT BETWEEN 255 AND 0 OR f3 BETWEEN 2 AND 2 AND
( f3 IS NULL OR f4 != 1 ) ;

returns no rows when executed with "union" in maria-5.1-wl21 and 1 row when executed with other optimizations and other trees. In 5.1 , the "union" optimization is not used.

test case:

CREATE TABLE t1 (f1 int,f2 int,f3 int,f4 int,
PRIMARY KEY (f1),KEY (f3),KEY (f4)) ;
INSERT IGNORE INTO t1 VALUES ('9','0','2','6'),('9930','0','0',NULL);

SET SESSION optimizer_switch='index_merge_intersection=off,index_merge_sort_intersection=off';
SET SESSION optimizer_switch='index_merge_union=on,index_merge_sort_union=off';

SELECT * FROM t1 FORCE KEY (PRIMARY,f3,f4)
WHERE f2 AND
( f3 IS NULL OR f1 = 7 )
AND f1 NOT BETWEEN 255 AND 0 OR f3 BETWEEN 2 AND 2 AND
( f3 IS NULL OR f4 != 1 ) ;

explain:

id: 1
select_type: SIMPLE
table: t1
type: index_merge
possible_keys: PRIMARY,f3,f4
key: f3,PRIMARY
key_len: 5,4
ref: NULL
rows: 2
Extra: Using union(f3,PRIMARY); Using where