Re: Make COPY format extendable: Extract COPY TO format implementations

Hi,

In <CAD21AoC19fV5Ujs-1r24MNU+hwTQUeZMEnaJDjSFwHLMMdFi0Q@mail.gmail.com>
  "Re: Make COPY format extendable: Extract COPY TO format implementations" on Wed, 25 Jun 2025 00:48:46 +0900,
  Masahiko Sawada <sawada.mshk@gmail.com> wrote:

>> >> It's natural to add more related APIs with this
>> >> approach. The single registration API provides one feature
>> >> by one operation. If we use the RegisterCopyRoutine() for
>> >> FROM and TO formats API, it's not natural that we add more
>> >> related APIs. In this case, some APIs may provide multiple
>> >> features by one operation and other APIs may provide single
>> >> feature by one operation. Developers may be confused with
>> >> the API. For example, developers may think "what does mean
>> >> NULL here?" or "can we use NULL here?" for
>> >> "RegisterCopyRoutine("new-format", NewFormatFromRoutine,
>> >> NULL)".
>> >
>> > We can document it in the comment for the registration function.
>>
>> I think that API that can be understandable without the
>> additional note is better API than API that needs some
>> notes.
> 
> I don't see much difference in this case.

OK. It seems that we can't agree on which API is better.

I've implemented your idea as the v42 patch set. Can we
proceed this proposal with this approach? What is the next
step?

> No. I think that if extensions are likely to support both
> CopyToRoutine and CopyFromRoutine in most cases, it would be simpler
> to register the custom format using a single API. Registering
> CopyToRoutine and CopyFromRoutine separately seems redundant to me.

I don't think so. In general, extensions are implemented
step by step. Extension developers will not implement
CopyToRoutine and CopyFromRoutine at once even if extensions
implement both of CopyToRoutine and CopyFromRoutine
eventually.

> Could you provide some examples? It seems to me that even if we
> provide the single API for the registration we can provide other APIs
> differently. For example, if we want to provide an API to register a
> custom option, we can provide RegisterCopyToOption() and
> RegisterCopyFromOption().

Yes. We can mix different style APIs. In general, consistent
style APIs is easier to use than mixed style APIs. If it's
not an important point in PostgreSQL API design, my point is
meaningless. (Sorry, I'm not familiar with PostgreSQL API
design.)

> My point is about the consistency of registration behavior. I think
> that we should raise an error if the custom format name that an
> extension tries to register already exists. Therefore I'm not sure why
> installing extension-A+B is okay but installing extension-C+A or
> extension-C+B is not okay? We can think that's an extension-A's choice
> not to implement CopyFromRoutine for the 'myformat' format so
> extension-B should not change it.

I think that it's the users' responsibility. I think that
it's more convenient that users can mix extension-A+B (A
provides only TO format and B provides only FROM format)
than users can't mix them. I think that extension-A doesn't
want to prohibit FROM format in the case. Extension-A just
doesn't care about FROM format.

FYI: Both of extension-C+A and extension-C+B are OK when we
update not raising an error existing format.


Thanks,
-- 
kou