Re: ALTER TYPE 2: skip already-provable no-work rewrites

On Sun, Jan 23, 2011 at 12:06:43PM -0500, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Sun, Jan 9, 2011 at 5:01 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> >> As unintended fallout, it's no longer an error to add oids or a column with a
> >> default value to a table whose rowtype is used in columns elsewhere. This seems
> >> best. Defaults on the origin table do not even apply to new inserts into such a
> >> column, and the rowtype does not gain an OID column via its table.
>
> > I think this should be split into two patches (we can discuss both on
> > this thread, no need to start any new ones), one that implements just
> > the above improvement and another that accomplishes the main purpose
> > of the patch.
>
> I haven't been paying much attention to this thread, but I happened to
> read the above, and quite frankly it scares the cr*p out of me. I don't
> believe that Noah even begins to be qualified to understand the
> implications of adding/removing an OID column, and I clearly remember
> the non-obvious bugs we've had in the past from that. Before this goes
> in I want to see a convincing explanation (not an unsupported assertion)
> why this isn't going to re-introduce those old bugs.

Turns out that we do set HEAP_HASOID and allocate space for an OID in the
composite-type datums. We don't actually assign an OID, and I can't see any way
to read it from the composite. It seems most consistent with the verdict of
commit 6d1e361 to continue rejecting these cases. Thanks for the heads-up.

> I'm also pretty unclear on why it's a good idea to let uses of a rowtype
> be different from the table on which it's allegedly based. To the
> extent that the current behavior allows that, isn't that a bug rather
> than a feature we should extend?

From the perspective of defining the behavior afresh, I'd agree. I haven't seen
any stirrings of actually implementing this, though. Like Robert, I'm doubting
there's a user benefit from rejecting these cases in preparation for the day
that they would actually require it.

nm