触发检查表中某些行中的某些值是否相等，如果不是则更新然后插入

Question

I'm having problem with trigger in psql which needs to check if some values already exist(sender, reciever, subject) in table and if they exist i only need to update one value(subject = RE: +subject) in that row, if not then I need to insert new row.

CREATE OR REPLACE FUNCTION check_theme()
RETURNS TRIGGER
AS $$
    DECLARE e BOOLEAN;
    BEGIN
        e := EXISTS(
            SELECT *
            FROM message
            WHERE sender = NEW.sender
              AND reciever = NEW.reciever
              AND subject = NEW.subject
            );
        IF NOT e THEN
            INSERT INTO message(sender, reciever, subject, text)
            VALUES(NEW.sender, NEW.reciever, NEW.subject, NEW.text);
        ELSE
            UPDATE message
            SET subject = 'Re: ' || NEW.subject
            WHERE sender = NEW.sender
              AND reciever = NEW.reciever
              AND subject = NEW.subject;
        END IF;
        RETURN NEW;

    END;
$$
LANGUAGE plpgsql;

CREATE TRIGGER theme_check
BEFORE INSERT OR UPDATE
ON message
FOR EACH ROW EXECUTE PROCEDURE check_theme(); 

So after I want to insert new row which already have same values for sender, reciever and subject trigger gets in infinitive loop.

What am I doing wrong?

Answer 1

After few days I have figured out that my knowledge of how triggers work was wrong.. so my mistake was that I was adding INSERT function but INSERT will be done anyway if there wasn't message with same sender, recipient and title.That's why I was always getting infinitive loop. So the code that will work is belove

CREATE OR REPLACE FUNCTION provjera_teme()
RETURNS TRIGGER
AS $$
    DECLARE postoji BOOLEAN;
    BEGIN
        postoji := EXISTS(
            SELECT poruka
            FROM poruka
            WHERE posiljatelj = NEW.posiljatelj
              AND primatelj = NEW.primatelj
              AND (naslov = NEW.naslov OR naslov LIKE 'Re: ' || NEW.naslov)
            );    
        IF postoji THEN
            UPDATE poruka
            SET naslov = 'Re: ' || naslov
            WHERE posiljatelj = NEW.posiljatelj
                  AND primatelj = NEW.primatelj
                  AND naslov = NEW.naslov AND naslov NOT LIKE 'Re%';
                RETURN NULL;
        END IF;
        RETURN NEW;
    END;
$$
LANGUAGE plpgsql;

CREATE TRIGGER tema_provjera
BEFORE INSERT
ON poruka
FOR EACH ROW EXECUTE PROCEDURE provjera_teme();