CREATE TABLE opa (
	id1 varchar( 128 ) NOT NULL,
	id2 varchar( 128 ) NOT NULL,
	PRIMARY KEY (id1, id2),
	KEY(id1),
	KEY(id2)
);

INSERT INTO opa (id1,id2) VALUES ('9','1');
INSERT INTO opa (id1,id2) VALUES ('2','1');
INSERT INTO opa (id1,id2) VALUES ('3','1');
INSERT INTO opa (id1,id2) VALUES ('4','2');
INSERT INTO opa (id1,id2) VALUES ('5','3');
INSERT INTO opa (id1,id2) VALUES ('6','4');

CREATE TABLE xex (
	id mediumint( 8 ) NOT NULL,
	symbol varchar( 128 ) NOT NULL,
	PRIMARY KEY (id),
	KEY (symbol)
);


INSERT INTO xex (id,symbol) VALUES ( 1 ,'1');
INSERT INTO xex (id,symbol) VALUES ( 2 ,'2');
INSERT INTO xex (id,symbol) VALUES ( 3 ,'3');
INSERT INTO xex (id,symbol) VALUES ( 4 ,'4');
INSERT INTO xex (id,symbol) VALUES ( 5 ,'5');
INSERT INTO xex (id,symbol) VALUES ( 6 ,'6');

EXPLAIN SELECT DISTINCT xex.*
FROM opa
INNER JOIN xex ON (opa.id1 = xex.symbol OR opa.id2 = xex.symbol)
WHERE opa.id2='1';

+-------+------+-----------------+------+---------+-------+------+-----------------------------+
| table | type | possible_keys   | key  | key_len | ref   | rows | Extra
               |
+-------+------+-----------------+------+---------+-------+------+-----------------------------+
| opa   | ref  | PRIMARY,id1,id2 | id2  |      128  | const |     2  | where used; U
sing temporary |
| xex   | ALL  | symbol          | NULL |    NULL | NULL  |     6  | where used
               |
+-------+------+-----------------+------+---------+-------+------+-----------------------------+

EXPLAIN SELECT DISTINCT xex.*
FROM opa
INNER JOIN xex ON (opa.id1 = xex.symbol OR opa.id2 = xex.symbol)
INNER JOIN xex2 ON (xex.id = xex2.id)
INNER JOIN xex3 ON (xex2.id = xex3.id) ................
WHERE opa.id2='1';