SELECT * FROM table1 WHERE (cols1=1000 OR cols2=1000) AND cols3=1;

+----+-------------+---------+------+-------------------------+------+---------+------+--------+-----------------------------+
| id | select_type | table   | type | possible_keys           | key  | key_len | ref  | rows   | Extra                       |
+----+-------------+---------+------+-------------------------+------+---------+------+--------+-----------------------------+
|  1 | SIMPLE      | matches | ALL  | cols3,cols1,cols2 | NULL | NULL    | NULL | 480000 | Using where; Using filesort |
+----+-------------+---------+------+-------------------------+------+---------+------+--------+-----------------------------+

SELECT * FROM (SELECT cols1,cols2,cols3 FROM table1  WHERE ((cols1=1000)OR(cols2=2768))) AS t1 WHERE t1.cols1=1;

+----+-------------+------------+-------------+------------------+------------------+---------+------+------+---------------------------------------------------------+
| id | select_type | table      | type        | possible_keys    | key              | key_len | ref  | rows | Extra                                                   |
+----+-------------+------------+-------------+------------------+------------------+---------+------+------+---------------------------------------------------------+
|  1 | PRIMARY     | <derived2> | ALL         | NULL             | NULL             | NULL    | NULL |  173 | Using where; Using filesort                             |
|  2 | DERIVED     | matches    | index_merge | cols1,cols2 | cols1,cols2 | 4,4     | NULL |  343 | Using union(cols1,cols2); Using where; Using index                |
+----+-------------+------------+-------------+------------------+------------------+---------+------+------+---------------------------------------------------------+