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

On Mon, Dec 11, 2023 at 7:19 PM Michael Paquier <michael@paquier.xyz> wrote:
>
> On Mon, Dec 11, 2023 at 10:57:15AM +0900, Masahiko Sawada wrote:
> > IIUC we cannot create two same name functions with the same arguments
> > but a different return value type in the first place. It seems to me
> > to be an overkill to change such a design.
>
> Agreed to not touch the logictics of LookupFuncName() for the sake of
> this thread.  I have not checked the SQL specification, but I recall
> that there are a few assumptions from the spec embedded in the lookup
> logic particularly when it comes to specify a procedure name without
> arguments.
>
> > Another idea is to encapsulate copy_to/from_handler by a super class
> > like copy_handler. The handler function is called with an argument,
> > say copyto, and returns copy_handler encapsulating either
> > copy_to/from_handler depending on the argument.
>
> Yep, that's possible as well and can work as a cross-check between the
> argument and the NodeTag assigned to the handler structure returned by
> the function.
>
> At the end, the final result of the patch should IMO include:
> - Documentation about how one can register a custom copy_handler.
> - Something in src/test/modules/, minimalistic still useful that can
> be used as a template when one wants to implement their own handler.
> The documentation should mention about this module.
> - No need for SQL functions for all the in-core handlers: let's just
> return pointers to them based on the options given.

Agreed.

> It would be probably cleaner to split the patch so as the code is
> refactored and evaluated with the in-core handlers first, and then
> extended with the pluggable facilities and the function lookups.

Agreed.

I've sketched the above idea including a test module in
src/test/module/test_copy_format, based on v2 patch. It's not splitted
and is dirty so just for discussion.


Regards,

--
Masahiko Sawada
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