Re: Followup Timestamp to timestamp with TZ conversion

On Fri, Jul 23, 2021 at 5:47 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > You seemed to think my previous comments about comparing opfamilies
> > were hypothetical but I think we actually already have the
> > optimization Peter wants, and it just doesn't apply in this case for
> > lack of hacks.
>
> Hmm. Note that what this is checking for is same operator *class* not
> same operator family (if it were doing the latter, Peter's case would
> already work). I think it has to do that. Extending my previous
> thought experiment about an unsigned integer type, if someone were to
> invent one, it would make a lot of sense to include it in integer_ops,
> and then the logic you suggest is toast. (Obviously, the cross-type
> comparison operators you'd need to write would have to be careful,
> but you'd almost certainly wish to write them anyway.)

Mumble. I hadn't considered that sort of thing. I assumed that when
the documentation and/or code comments talked about a compatible
notion of equality, it was a strong enough notion of "compatible" to
preclude this sort of case. I'm not really sure why we shouldn't think
of it that way; the example you give here is reasonable, but
artificial.

> Given that we require the non-cross-type members of an opclass to be
> immutable, what this is actually doing may be safe. At least I can't
> construct a counterexample after five minutes' thought. On the other
> hand, I'm also a bit confused about how it ever succeeds at all.
> If we've changed the heap column's type, it should not be using the
> same opclass anymore (unless the opclass is polymorphic, but that
> case is rejected too). I'm suspicious that this is just an expensive
> way of writing "we can only preserve indexes that aren't on the
> column that changed type".

Well, you can change just the typemod, for example, which was a case
that motivated this work originally. People wanted to be able to make
varchar(10) into varchar(20) without doing a ton of work, and I think
this lets that work. I seem to recall that there are at least a few
cases that involve actually changing the type as well, but at 6pm on a
Friday evening when I haven't looked at this in years, I can't tell
you what they are off the top of my head.

--
Robert Haas
EDB: http://www.enterprisedb.com