Thread: Odd plpgsql behaviour

On 7.4:

This is what we wanted to do:

IF TG_OP = 'INSERT' OR (TG_OP = 'UPDATE' AND NEW.name != OLD.name) THEN EXECUTE x;
END IF;

However, we had to write it like this:

IF TG_OP = 'INSERT' THEN EXECUTE x;
ELSIF TG_OP = 'UPDATE' AND NEW.name != OLD.name THEN EXECUTE x;
END IF;

Because in the first case it would complain that OLD.name wasn't 
defined, if the trigger was NOT an update.

OK, but the second case works??!?!  Is this a weird peculiarity of the 
pl/pgsql lazy evaluation rules?  Why doesn't the first one work if the 
second one does?

Chris

On Mon, 15 Nov 2004 15:12:24 +0800, Christopher Kings-Lynne
<chriskl@familyhealth.com.au> wrote:
> On 7.4:
> 
> This is what we wanted to do:
> 
> IF TG_OP = 'INSERT' OR (TG_OP = 'UPDATE' AND NEW.name != OLD.name) THEN
>   EXECUTE x;
> END IF;
> 
> However, we had to write it like this:
> 
> IF TG_OP = 'INSERT' THEN
>   EXECUTE x;
> ELSIF TG_OP = 'UPDATE' AND NEW.name != OLD.name THEN
>   EXECUTE x;
> END IF;
> 
> Because in the first case it would complain that OLD.name wasn't
> defined, if the trigger was NOT an update.
> 
> OK, but the second case works??!?!  Is this a weird peculiarity of the
> pl/pgsql lazy evaluation rules?  Why doesn't the first one work if the
> second one does?

IIRC, the reason for this is that the entire IF test is passed to the
SQL engine as a SELECT statement after replacing the TG_* identifiers
with their respective values.

Your first example is essentially
  IF (SELECT (TG_OP = 'INSERT' OR (TG_OP = 'UPDATE' AND NEW.name !=
OLD.name) IS TRUE) ...

In this case, since OLD.name does not exist during INSERT it cannot be
replaced.  Perhaps someone else can shed a little more light on this.

-- 
Mike Rylander
mrylander@gmail.com
GPLS -- PINES Development
Database Developer

Mike Rylander <mrylander@gmail.com> writes:
> IIRC, the reason for this is that the entire IF test is passed to the
> SQL engine as a SELECT statement after replacing the TG_* identifiers
> with their respective values.
> In this case, since OLD.name does not exist during INSERT it cannot be
> replaced.  Perhaps someone else can shed a little more light on this.

Nope, that's about all there is to say about it ...
        regards, tom lane