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

On Mon, Mar 04, 2024 at 02:11:08PM +0900, Sutou Kouhei wrote:
> If this is a blocker of making COPY format extendable, can
> we defer moving the existing text/csv/binary format
> implementations to Copy{From,To}Routine for now as Michael
> suggested to proceed making COPY format extendable? (Can we
> add Copy{From,To}Routine without changing the existing
> text/csv/binary format implementations?)

Yeah, I assume that it would be the way to go so as we don't do any
dispatching in default cases. A different approach that could be done
is to hide some of the parts of binary and text/csv in inline static
functions that are equivalent to the routine callbacks. That's
similar to the previous versions of the patch set, but if we come back
to the argument that there is a risk of blocking optimizations of more
of the local areas of the per-row processing in NextCopyFrom() and
CopyOneRowTo(), what you have sounds like a good balance.

CopyOneRowTo() could do something like that to avoid the extra
indentation:
if (cstate->routine)
{
cstate->routine->CopyToOneRow(cstate, slot);
MemoryContextSwitchTo(oldcontext);
return;
}

NextCopyFrom() does not need to be concerned by that.

> I attach a patch for it.

> There is a large hunk for CopyOneRowTo() that is caused by
> indent change. I also attach "...-w.patch" that uses "git
> -w" to remove space only changes. "...-w.patch" is only for
> review. We should use .patch without -w for push.

I didn't know this trick. That's indeed nice.. I may use that for
other stuff to make patches more presentable to the eyes. And that's
available as well with `git diff`.

If we basically agree about this part, how would the rest work out
with this set of APIs and the possibility to plug in a custom value
for FORMAT to do a pg_proc lookup, including an example of how these
APIs can be used?
--
Michael