Re: BUG #17689: Two UPDATE operators in common table expressions (CTE) perform not as expected

Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> On 2022-Nov-18, Marko Tiikkaja wrote:
>> This is a documented limitation:
>>> Trying to update the same row twice in a single statement is not
>>> supported.

> I wonder if we should try to detect the case, and raise an error instead
> of it resulting in undefined behavior.

My recollection is that that is really fallout from an ancient and
intentional executor behavior, that we have to ignore multiple updates
in order to not get into infinite loops.  See comment about the
"Halloween problem" in nodeLockRows.c.  (I'm pretty sure there were once
more comments about that, somewhere closer to ExecUpdate/ExecDelete ---
this all dates back to Berkeley.)