Deleting rows from a table not contained in another table

I want to reimplement

DELETE FROM foo;
INSERT INTO foo SELECT * FROM bar;

in a way which does not touch rows which are not modified (mainly to
avoid locking issues). I've come up with this:

DELETE FROM foo WHERE NOT EXISTS
(SELECT * FROM bar WHERE foo.* IS NOT DISTINCT FROM bar.*);
INSERT INTO foo SELECT * FROM bar EXCEPT SELECT * FROM foo;

The problem is that the plan for the DELETE doesn't look pretty at all:

QUERY PLAN
-----------------------------------------------------------------------
Nested Loop Anti Join (cost=313.36..181568.96 rows=1 width=6)
Join Filter: (NOT (foo.* IS DISTINCT FROM bar.*))
-> Seq Scan on foo (cost=0.00..293.05 rows=20305 width=38)
-> Materialize (cost=313.36..516.40 rows=20305 width=32)
-> Seq Scan on bar (cost=0.00..293.05 rows=20305 width=32)
(5 rows)

Is there some way to turn this into a merge join, short of introducing
primary keys and using them to guide the join operation?

--
Florian Weimer <fweimer(at)bfk(dot)de>
BFK edv-consulting GmbH http://www.bfk.de/
Kriegsstraße 100 tel: +49-721-96201-1
D-76133 Karlsruhe fax: +49-721-96201-99