LEFT JOIN and handling NULL Values
This may be the most efficient solution:

SELECT A, t1.B, t2.C
FROM   table1 t1
LEFT   JOIN table2 t2 USING (A)
ORDER  BY t2.C NULLS FIRST;


NULL values are sorted last by default. You can reverse that with NULLS FIRST. See:


PostgreSQL sort by datetime asc, null first?