Here's the solution I've ended up with, based on your suggestion:
CREATE FUNCTION skip_update() RETURNS trigger AS $$
BEGIN
RETURN NULL;
END;
$$ LANGUAGE plpgsql;
CREATE FUNCTION fail_update() RETURNS trigger AS $$
BEGIN
RAISE EXCEPTION 'can''t modify row values for existing primary
+ key';
END;
$$ LANGUAGE plpgsql;
CREATE TABLE Dummy_Test (
Timestamp TIMESTAMP WITH TIME ZONE PRIMARY KEY,
Foo TEXT,
Bar TEXT
);
CREATE TRIGGER Dummy_Test_dupe BEFORE UPDATE ON Dummy_Test FOR EACH RO
+W
WHEN ( OLD.Timestamp IS NOT DISTINCT FROM NEW.Timestamp AND OLD.Fo
+o IS NOT DISTINCT FROM NEW.Foo AND OLD.Bar IS NOT DISTINCT FROM NEW.B
+ar )
EXECUTE FUNCTION skip_update();
CREATE TRIGGER Dummy_Test_modify BEFORE UPDATE ON Dummy_Test FOR EACH
+ROW
WHEN ( OLD.Foo IS DISTINCT FROM NEW.Foo OR OLD.Bar IS DISTINCT FRO
+M NEW.Bar )
EXECUTE FUNCTION fail_update();
INSERT INTO Dummy_Test (Timestamp,Foo,Bar) VALUES ('2020-01-02 12:34',
+'Hello','World') ON CONFLICT ON CONSTRAINT Dummy_Test_pkey DO UPDATE
+SET Foo=EXCLUDED.Foo, Bar=EXCLUDED.Bar;
INSERT INTO Dummy_Test (Timestamp,Foo,Bar) VALUES ('2020-01-02 12:34',
+'Hello','World') ON CONFLICT ON CONSTRAINT Dummy_Test_pkey DO UPDATE
+SET Foo=EXCLUDED.Foo, Bar=EXCLUDED.Bar;
INSERT INTO Dummy_Test (Timestamp,Foo,Bar) VALUES ('2020-01-02 12:34',
+'Hello','abcde') ON CONFLICT ON CONSTRAINT Dummy_Test_pkey DO UPDATE
+SET Foo=EXCLUDED.Foo, Bar=EXCLUDED.Bar;
The last statement will fail, which is exactly what I wanted. The function FUNCTION skip_update() and TRIGGER Dummy_Test_dupe really only prevent the UPDATE from happening, they can be omitted - I haven't yet tested whether it's more performant to have the TRIGGER or the UPDATE fire. In any case, thank you very much!