Inserting a COALESCE(NULL,default)

Question

I have tables that use UUIDs. I want to be able to insert a new row with or without a UUID as sometimes the client will generate the UUID other times it won\'t.

Each

Answer 1

There's no way to re-use the defined default on the column. The default is only there to define what happens if an INSERT doesn't specify a value. By this definition a null value is still "specified" and therefore default can't be used.

Your comment that someone might not use the function indicates that a trigger is better for your requirements than a simple function.

https://www.postgresql.org/docs/current/static/plpgsql-trigger.html

CREATE OR REPLACE FUNCTION default_id() RETURNS TRIGGER AS $default_id$
    BEGIN       
        IF (NEW.id IS NULL) THEN
            NEW.id := gen_random_uuid();
        END IF;
        RETURN NEW;
    END;
$default_id$ LANGUAGE plpgsql;

CREATE TRIGGER default_id_trigger
BEFORE INSERT OR UPDATE ON person
    FOR EACH ROW EXECUTE PROCEDURE default_id();

---

If you do want to do this with a function then the simplest way is just to assign the value before inserting:

CREATE OR REPLACE FUNCTION create_person(
    id UUID
) RETURNS BOOLEAN LANGUAGE plpgsql SECURITY DEFINER AS $$
BEGIN
    IF id IS NULL THEN
        id := gen_random_uuid();
    END IF;
    -- OR
    -- id := coalesce(id, gen_random_uuid());
    INSERT INTO person( id )
    VALUES (id);
    RETURN FOUND;
END;
$$;