What's faster - full outer join or sum a union

Question

Hello,
what do you think it's the best way to bring two tables together? Full outer join (1) or sum over a union (2)?
1.
select a.id nvl(a.x,0) ,nvl(b.y,0) from
a full outer join b
where a.id=b.id
2.
select id, sun(x), sum(y) from
(select id, x,0 y from a
union all
select id, 0 x, y from b )
group by id;
The ids are primary keys and ca. 80% of ids in a are found in b and vice versa.
Both tables have about 20 mio datasets.
Thank for thinking about
Stephan

Answer 1

Like so many things in Oracle, the answer is it depends. Various factors such as resources available (memory, CPU, disk speed etc.), size of the tables, and the indexes available will all influence the performance of a given query.
Having said that, my guess would be that the UNION ALL query would be faster than the full outer join, since ther is less work involved. However, the optimized may be smart enough to re-write the full outer join as a union all.
Another approach which might be worth benchmarking is:
SELECT id, SUM(x), SUM(y)
FROM (SELECT id, SUM(x), 0 y
      FROM a
      GROUP BY id
      UNION ALL
      SELECT id, 0, SUM(y) y
      FROM b
      GROUP BY id)HTH
John