I wrote:
> Neel Patel <neel.patel@enterprisedb.com> writes:
>> Is this error message expected or what should be the behaviour ?
> It's certainly a bug.
Oh, no, I take that back: it's not a bug, you're just abusing
pg_get_expr() to try to do something it can't do, which is make
sense of an expression involving more than one relation.
(OLD and NEW are different relations in a trigger WHEN clause.)
You can use pg_get_triggerdef() to decompile a trigger WHEN clause,
although that might do more than you want.
Not sure if there's any value in trying to make the failure
message more user-friendly. You can get weird errors by
misusing pg_get_expr() in other ways too, such as giving it
the wrong relation OID.
regards, tom lane