I think the "best" approach (if that is supported by your DBMS) are ON UPDATE triggers, or if that import is a one time thing only, creating a unique key across all columns:
create or replace function skip_insert() returns trigger language plpg
+sql as $$
begin
return null;
end $$;
create or replace function raise_exception() returns trigger language
+plpgsql as $$
begin
RAISE EXCEPTION 'UPDATE changing row values not allowed';
end $$;
CREATE TRIGGER mytrigger ON records INSTEAD OF UPDATE
WHEN old.PriKey = new.PriKey and old.Foo=new.Foo and old.Bar=new.B
+ar
FOR EACH ROW
EXECUTE PROCEDURE skip_insert()
CREATE TRIGGER mytrigger ON records INSTEAD OF UPDATE
WHEN old.PriKey = new.PriKey and (old.Foo!=new.Foo or old.Bar!=new
+.Bar)
FOR EACH ROW
EXECUTE PROCEDURE raise_exception()
... but this approach does not handle NULL values.
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