improve union support

[siddontang]

SELECT anon_1.id, anon_1.x, anon_1.y 
FROM ((SELECT some_table.id, some_table.x, some_table.y 
FROM some_table 
WHERE some_table.id = 2 ORDER BY some_table.id 
 LIMIT 1) UNION (SELECT some_table.id, some_table.x, some_table.y 
FROM some_table 
WHERE some_table.id = 3 ORDER BY some_table.id 
 LIMIT 1)) AS anon_1 ORDER BY anon_1.id 
 LIMIT 2