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

Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> writes:
> At Tue, 26 Nov 2019 10:49:11 -0500, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote in
>> Hmm, interesting. IMO, that *should* have thrown an error, but of
>> course not that one. The ADD COLUMN operations are all processed
>> in parallel, so it's not okay for one of them to have a GENERATED
>> expression that refers to another one of the new columns. But you
>> should have gotten a "no such column" type of error, not a run-time
>> cross-check failure.

> Something like this works?

I started to look at this, but it felt a bit brute-force to me.
After awhile I began to think that my offhand comment above was
wrong --- why *shouldn't* this case work? When we insert or
update a tuple, we expect that GENERATED columns should be
computed based on the new tuple values, so why is the executor
evidently evaluating them based on the old tuple?

That thought soon led me to realize that there's an adjacent
bug that this patch fails to fix:

regression=# create table foo (f1 int);
CREATE TABLE
regression=# insert into foo values(1),(2);
INSERT 0 2
regression=# alter table foo alter column f1 type float8, add column f2 int generated always as (f1 * 2) stored;
ERROR: attribute 1 of type foo has wrong type
DETAIL: Table has type integer, but query expects double precision.

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.

regards, tom lane