Re: ERROR: attribute number 6 exceeds number of columns 5

I wrote:
> So I believe that the real problem here is that the executor is
> evaluating GENERATED expressions at the wrong time. It's evaluating
> them against the pre-conversion tuples when it should be evaluating
> them against the post-conversion tuples. We need to go fix that,
> rather than inserting arbitrary restrictions in the DDL code.

I looked at that more closely, and realized that blaming the executor
is wrong: the real issue is that ALTER TABLE itself supposes that it
need only evaluate expressions against the old tuple. That's easy
to fix with a bit more code though. I propose the attached.

(Note that this should also allow relaxing the existing implementation
restriction against changing types of columns that GENERATED columns
depend on: all we have to do is re-parse the generation expression
and schedule it for evaluation. I've not looked into that, and it
doesn't seem like a bug fix anyway.)

regards, tom lane