Keep default behavior when new trigger is inserted SQL - postgresql

Question

Let's say I have contraints on a table. When I try to insert something there which doesn't have the correct data, I get an error in the console saying that this and that IC was violated.
ERROR: null value in column "column" violates not-null constraint
DETAIL: Failing row contains (0, null, null, null, null).
Now if I add a this:
CREATE OR REPLACE FUNCTION trigger_function()
RETURNS trigger AS $BODY$
BEGIN
raise notice 'test';
return null;
END;
$BODY$ LANGUAGE plpgsql;
create trigger test_trigger
before update or insert
on tablename
for each row
execute procedure trigger_function();
And execute the same thing as before I instead get this:
NOTICE: test
INSERT 0 0
I know that this trigger isn't really useful but I'm still learning how they work.
Is it possible to keep the previous error messages, and add whatever I did in the trigger_function after it? Why does it replace all the default messages?

Answer 1

Quote from the manual
Row-level triggers fired BEFORE can return null to signal the trigger manager to skip the rest of the operation for this row (i.e., subsequent triggers are not fired, and the INSERT/UPDATE/DELETE does not occur for this row)
(emphasis mine)
Because your trigger returns NULL the insert is never actually done and thus no constraint can be violated.
The output in the psql console
INSERT 0 0
tells you that no rows have been inserted. The second value is the number of rows inserted.
If you want a trigger that "does nothing", you need to use return new
CREATE OR REPLACE FUNCTION trigger_function()
RETURNS trigger
AS $BODY$
BEGIN
raise notice 'test';
return new; --<< here
END;
$BODY$ LANGUAGE plpgsql;