This query:

SELECT *
FROM ( SELECT * FROM t2 ) AS alias1
WHERE EXISTS (
SELECT t1.a
FROM t3
RIGHT JOIN t1
ON ( t3.d = t1.a )
WHERE t1.c < alias1.d
);

returns no rows when executed with derived_merge=on, whereas with derived_merge=off, in mysql-5.5 and in postgresql it returns:

----------+

----------+

----------+

explain:

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

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

minimal optimizer_switch: derived_merge=on
full optimizer_switch: index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,index_condition_pushdown=on,derived_merge=on,derived_with_keys=off,firstmatch=off,loosescan=off,materialization=off,in_to_exists=on,semijoin=off,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=off,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=off,semijoin_with_cache=off,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on

test case:

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 ( a int NOT NULL , e int, c varchar(32)) ;
INSERT INTO t1 VALUES (28,9,'j');

DROP TABLE IF EXISTS t2;
CREATE TABLE t2 ( c int, d varchar(32)) ;
INSERT INTO t2 VALUES (0,'r'),(0,'p');

DROP TABLE IF EXISTS t3;
CREATE TABLE t3 (d int);
INSERT INTO t3 VALUES (0),(0);

SET SESSION optimizer_switch='derived_merge=on';
SELECT *
FROM ( SELECT * FROM t2 ) AS alias1
WHERE EXISTS (
SELECT t1.a
FROM t3
RIGHT JOIN t1
ON ( t3.d = t1.a )
WHERE t1.c < alias1.d
);

bzr version-info
revision-id: <email address hidden>
date: 2011-08-09 18:34:26 +0300
build-date: 2011-08-10 13:26:22 +0300
revno: 3148
branch-nick: maria-5.3