Depending on the version of Pg there are two possible solutions to
this problem. The first (old solution) that really only works well
one row at a time is to do a stored procedure that does something
like:
update foo set bar = baz where id = in_id
if not found
insert into foo (bar) values (baz)
end if;
The newer way, which can be done in SQL with Pg 9.1 is to use writable
common table expressions. See
http://vibhorkumar.wordpress.com/2011/10/26/upsertmerge-using-writable-cte-in-postgresql-9-1/
for an example by Vibhor Kumar.
Best Wishes,
Chris Travers
