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

On Mon, Mar 11, 2024 at 8:56 AM Sutou Kouhei <kou@clear-code.com> wrote:
>
> Hi,
>
> In <CACJufxEgn3=j-UWg-f2-DbLO+uVSKGcofpkX5trx+=YX6icSFg@mail.gmail.com>
>   "Re: Make COPY format extendable: Extract COPY TO format implementations" on Mon, 11 Mar 2024 08:00:00 +0800,
>   jian he <jian.universality@gmail.com> wrote:
>
> > Hi, here are my cents:
> > Currently in v17, we have 3 extra functions within DoCopyTo
> > CopyToStart, one time, start, doing some preliminary work.
> > CopyToOneRow, doing the repetitive work, called many times, row by row.
> > CopyToEnd, one time doing the closing work.
> >
> > seems to need a function pointer for processing the format and other options.
> > or maybe the reason is we need a one time function call before doing DoCopyTo,
> > like one time initialization.
>
> I know that JSON format wants it but can we defer it? We can
> add more options later. I want to proceed this improvement
> step by step.
>
> More use cases will help us which callbacks are needed. We
> will be able to collect more use cases by providing basic
> callbacks.

I guess one of the ultimate goals would be that COPY can export data
to a customized format.
Let's say the customized format is "csv1", but it is just analogous to
the csv format.
people should be able to create an extension, with serval C functions,
then they can do `copy (select 1 ) to stdout (format 'csv1');`
but the output will be exact same as `copy (select 1 ) to stdout
(format 'csv');`

In such a scenario, we require a function akin to ProcessCopyOptions
to handle situations
where CopyFormatOptions->csv_mode is true, while the format is "csv1".

but CopyToStart is already within the DoCopyTo function, so you do
need an extra function pointer?
I do agree with the incremental improvement method.