Check for value with current_setting()

Question

I'm trying to work with current_setting().

I came up with this:

CREATE OR REPLACE FUNCTION process_audit() RETURNS TRIGGER AS $audit$
    DECLARE
        user_id integer;
    BEGIN
        BEGIN
            user_id := current_setting('hws.current_user_id');
        EXCEPTION WHEN OTHERS THEN
            user_id := NULL;
        END;
        ...
        RETURN NULL;
   END;
$audit$ LANGUAGE plpgsql;

The setting is set via:

SELECT set_config('hws.current_user_id', '5', true); -- true = local setting -> only visible in current transaction

The problem is, that current_setting() throws an exception if the value is not valid. I don't want to use EXCEPTION because I read that exception blocks are expensive.

Is there a way to check if the setting has a value without using exceptions?

Btw: I also tried to read from pg_settings but that doesn't seem to work with local settings.

Answer 1

9.6 and newer:

PostgreSQL (9.6+) supports current_setting('setting_name', 't') to fetch a setting and return NULL if it's unset. you can combine this with coalesce to supply a default.

9.5 and older:

Per the question, you can do it with a plpgsql function that uses a BEGIN ... EXCEPTION handler, if you don't mind the performance hit and clumsiness. But there's no built-in support.

Answer 2

Shouldn't you also make a cast? As follows,

user_id := current_setting('hws.current_user_id')::integer;