UPDATE statement (merge table content)

Question

I have two tables t1 and t2 with the same structure: an unique primary key and some arbitrary fields. The tables look something like:
pk number
f1 varchar2(10)
f2 varchar2(10)
f3 varchar2(10)
I have some "new" data in the records in t2 that I would like to update the corresponding rows in t1 with.
On a Microsoft SQL server I could use a SQL statement like:
UPDATE t1
SET t1.f1 = new.f1, t1.f2 = new.f2
FROM t1, t2 new
WHERE new.pk = t1.pk;
But this doesn't work with Oracle (8.1.7). Instead I have come up with something like:
UPDATE t1 old
SET (f1, f2) =
(SELECT f1, f2 FROM t2 new WHERE new.pk = old.pk)
WHERE old.pk IN (SELECT pk FROM t2);
The problem here is that the Oracle version performs very poorly (it's obvious why), whereas the MS SQL server version works like a charm.
There must be something I'm missing here - can anybody please enlithen me.
How do you merge the contents of two tables on Oracle.
Regards,
Tom Bjerre
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Answer 1

I have had two sugestions. Stephan Born from Germany suggests:
update t1 old
set ( f1, f2 ) = ( select f1, f2
from t2 new
where new.pk = old.pk
where exits ( select x
from t2 new
where new.pk = old.pk
When there exists an index for t2.pk this should result in one full-scan on t1 and two index-scans on t2.
Another suggestion from Ian Ledzion was to combine both tables in the first select:
UPDATE (SELECT old.f1 old_f1, new.f1 new_f1, old.f2 old_f2, new.f2 new_f2
FROM t1 old, t2 new
WHERE (old.id = new.id)) a
SET a.old_f1 = a.new_f1, a.old_f2 = a.new_f2;
Both suggestions seems to be faster than my first attempt (usning the IN clause). Can anybody tell which is best? It could depend on the relative size of t1 and t2 (number of records)
Regards, Tom Bjerre
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