Thread: plpgsql lacks generic identifier for record in triggers...

Now that pgmemcache is getting more use, I've heard a couple of groans 
regarding the need to have two functions with exactly the same code 
body.  This is necessary because there is no generic way of handling 
NEW/OLD.  For example:

db=# CREATE FUNCTION schma.tbl_ins_upd() RETURNS TRIGGER AS 'BEGIN        EXECUTE public.mc_init();        EXECUTE
public.mc_delete(''mc_key'');       RETURN NEW;
 
END;' LANGUAGE 'plpgsql';
db=# CREATE FUNCTION schma.tbl_del() RETURNS TRIGGER AS 'BEGIN        EXECUTE public.mc_init();        EXECUTE
public.mc_delete(''mc_key'');       RETURN OLD;
 
END;' LANGUAGE 'plpgsql';
db=# CREATE TRIGGER tbl_ins_upd_trg AFTER INSERT OR UPDATE ON schma.tbl 
FOR EACH ROW EXECUTE PROCEDURE schma.tbl_ins_upd();
db=# CREATE TRIGGER tbl_del_trg AFTER DELETE ON schma.tbl FOR EACH ROW 
EXECUTE PROCEDURE schma.tbl_del();

It's be nice if there was a generic return type so that one could 
collapse those two functions and trigger creation statements into one 
function and one trigger.  Something like:

db=# CREATE FUNCTION schma.tbl_inval() RETURNS TRIGGER AS 'BEGIN        EXECUTE public.mc_init();        EXECUTE
public.mc_delete(''mc_key'');       RETURN ROW;
 
END;' LANGUAGE 'plpgsql';
db=# CREATE TRIGGER tbl_inval_trg AFTER INSERT OR UPDATE OR DELETE ON 
schma.tbl FOR EACH ROW EXECUTE PROCEDURE schma.tbl_inval();


pgmemcache has pushed this to the surface as a problem that otherwise 
wouldn't exist.  That said, plpgsql's semantics are clearly the issue 
here as it's a syntax problem.  ROW being an alias for NEW in the 
INSERT and UPDATE case, and OLD in the DELETE case.  Thoughts?  Would a 
patch be accepted that modified plpgsql's behavior to include a new 
predefined alias?  Better yet, could TRIGGER functions be allowed to 
return nothing (ala VOID)?  For example:

db=# CREATE FUNCTION schma.tbl_inval() RETURNS TRIGGER AS 'BEGIN        EXECUTE public.mc_init();        EXECUTE
public.mc_delete(''mc_key'');       RETURN;
 
END;' LANGUAGE 'plpgsql';
db=# CREATE TRIGGER tbl_inval_trg AFTER INSERT OR UPDATE OR DELETE ON 
schma.tbl FOR EACH ROW EXECUTE PROCEDURE schma.tbl_inval();

Which would tell the backend to assume that the row wasn't changed and 
proceed with its handling.  This is the preferred approach, IMHO... but 
I think is the hardest to achieve (I haven't looked to see what'd be 
involved yet).

Enjoy your T-Day commute if you haven't yet.  -sc

-- 
Sean Chittenden

> Now that pgmemcache is getting more use, I've heard a couple of groans 
> regarding the need to have two functions with exactly the same code 
> body.  This is necessary because there is no generic way of handling 
> NEW/OLD.  For example:

[snip]  Err... wait, this is a classic case of send first then 
finishing to pondering the gripe.

> db=# CREATE FUNCTION schma.tbl_inval() RETURNS TRIGGER AS 'BEGIN
>         EXECUTE public.mc_init();
>         EXECUTE public.mc_delete(''mc_key'');
>         RETURN ROW;
> END;' LANGUAGE 'plpgsql';
> db=# CREATE TRIGGER tbl_inval_trg AFTER INSERT OR UPDATE OR DELETE ON 
> schma.tbl FOR EACH ROW EXECUTE PROCEDURE schma.tbl_inval();

A statement trigger should be used instead since the return value is 
ignored (and NULL can be used to satisfy the need for return to 
actually return something).  When updating dynamic keys, you always 
need to be explicit regarding NEW/OLD to get the data version, but for 
static keys, statement triggers are the way to go.  Ex:

> db=# CREATE FUNCTION schma.tbl_inval() RETURNS TRIGGER AS 'BEGIN
>         EXECUTE public.mc_init();
>         EXECUTE public.mc_delete(''mc_key'');
>         RETURN NULL;
> END;' LANGUAGE 'plpgsql';
> db=# CREATE TRIGGER tbl_inval_trg AFTER INSERT OR UPDATE OR DELETE ON 
> schma.tbl FOR EACH STATEMENT EXECUTE PROCEDURE schma.tbl_inval();

Very nice.  -sc

-- 
Sean Chittenden

Sean Chittenden <sean@chittenden.org> writes:
> ... Better yet, could TRIGGER functions be allowed to 
> return nothing (ala VOID)?
> Which would tell the backend to assume that the row wasn't changed and 
> proceed with its handling.  This is the preferred approach, IMHO... but 
> I think is the hardest to achieve (I haven't looked to see what'd be 
> involved yet).

plperl is doing it that way, so I don't see why plpgsql couldn't allow
it.
        regards, tom lane

>
> db=# CREATE FUNCTION schma.tbl_ins_upd() RETURNS TRIGGER AS 'BEGIN
>         EXECUTE public.mc_init();
>         EXECUTE public.mc_delete(''mc_key'');
>         RETURN NEW;
> END;' LANGUAGE 'plpgsql';
> db=# CREATE FUNCTION schma.tbl_del() RETURNS TRIGGER AS 'BEGIN
>         EXECUTE public.mc_init();
>         EXECUTE public.mc_delete(''mc_key'');
>         RETURN OLD;
> END;' LANGUAGE 'plpgsql';

could this be used?

CREATE FUNCTION schma.tbl_ins_upd() RETURNS TRIGGER AS 'BEGIN
       EXECUTE public.mc_init();       EXECUTE public.mc_delete(''mc_key'');   if TG_OP = ''INSERT'' or TG_OP =
''UPDATE''then       RETURN NEW;   else       RETURN OLD;   end if;   
 
END;' LANGUAGE 'plpgsql';


regards

Laser