SELECT CASE WHEN useridfrom > useridto
                        THEN  '' || useridfrom || '-' || useridto 
                        ELSE  '' || useridto || '-' || useridfrom  END AS convo_signature, m.smallmessage, m.id, m.useridto, m.useridfrom, m.timeread
                       FROM (SELECT
                                id, useridfrom, useridto, smallmessage, 0 as timeread
                            FROM mdl_message
                            WHERE
                                (id = 4 AND timecreated = 1486606496)
                            UNION ALL
                            SELECT
                                id, useridfrom, useridto, smallmessage, timeread
                            FROM mdl_message_read
                            WHERE
                                (id = 4 AND timecreated = 1486606496)) m
								
Empty set (1.89 sec)

+----+--------------+------------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | PRIMARY      | <derived2> | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL                                                |
|  2 | DERIVED      | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | no matching row in const table                      |
|  3 | UNION        | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |
| NULL | UNION RESULT | <union2,3> | ALL  | NULL          | NULL | NULL    | NULL | NULL | Using temporary                                     |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------------------------------------------+
4 rows in set (0.16 sec)