Обсуждение: Short circuit operations in plpgsql?

I have some triggers. They need to do stuff when a new row is inserted
or when the value of a particular field changes.

So, something like:

  IF TG_OP='INSERT' OR NEW.foo != OLD.foo THEN
    -- do stuff
  END IF;
  RETURN NEW;

Is there anything that guarantees that the NEW.foo != OLD.foo won't
be evaluated (and hence fail, as there is no OLD) on an insert?

Cheers,
  Steve

Steve Atkins <steve@blighty.com> writes:
>   IF TG_OP='INSERT' OR NEW.foo != OLD.foo THEN
>     -- do stuff

> Is there anything that guarantees that the NEW.foo != OLD.foo won't
> be evaluated (and hence fail, as there is no OLD) on an insert?

The only way is to split the expression into two IF commands.  The real
problem is not with the evaluation per se, but with plpgsql having to
pass down all the variable values used in the expression before
evaluation can start.

            regards, tom lane