Re: Potentially undocumented behaviour change in Postgres 11concerning OLD record in an after insert trigger

Kristjan Tammekivi <kristjantammekivi@gmail.com> writes:
> I've noticed a change in the behaviour in triggers / hstores in Postgres
> 11.1 when compared to Postgres 10.5.
> [ reference to OLD in an insert trigger doesn't throw error anymore ]

Hmm.  This seems to be a side effect of the changes we (I) made in v11
to rationalize the handling of NULL vs ROW(NULL,NULL,...) composite
values in plpgsql.  The "unassigned" trigger row variables are now
acting as though they are plain NULL values.  I'm not sure now whether
I realized that this would happen --- it looks like there are not and
were not any regression test cases that would throw an error for the
disallowed-reference case, so it's quite possible that it just escaped
notice.

The old behavior was pretty darn squirrely; in particular it would let
you parse but not execute a reference to "OLD.column", a behavior that
could not occur for any other composite variable.  Now that'll just
return NULL.  In a green field I don't think there'd be complaints
about this behavior --- I know lots of people have spent considerable
effort programming around the other behavior.

While I haven't looked closely, I think duplicating the old behavior
would require adding a special-purpose flag to plpgsql DTYPE_REC
variables, which'd cost a little performance (extra error checks
in very hot code paths) and possibly break compatibility with
pldebugger, if there's been a v11 release of that.

So I'm a bit inclined to accept this behavior change and adjust
the documentation to say that OLD/NEW are "null" rather than
"unassigned" when not relevant.

Thoughts?

                        regards, tom lane