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

Sutou Kouhei <kou@clear-code.com>

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From: Sutou Kouhei <kou@clear-code.com>
To: zhjwpku@gmail.com
Cc: sawada.mshk@gmail.com, andrew@dunslane.net, michael@paquier.xyz, nathandbossart@gmail.com, pgsql-hackers@postgresql.org
Date: 2024-01-29T06:03:32Z
Lists: pgsql-hackers 
Hi,

In <CAEG8a3JDPks7XU5-NvzjzuKQYQqR8pDfS7CDGZonQTXfdWtnnw@mail.gmail.com>
  "Re: Make COPY format extendable: Extract COPY TO format implementations" on Sat, 27 Jan 2024 14:15:02 +0800,
  Junwang Zhao <zhjwpku@gmail.com> wrote:

> I have been working on a *COPY TO JSON* extension since yesterday,
> which is based on your V6 patch set, I'd like to give you more input
> so you can make better decisions about the implementation(with only
> pg-copy-arrow you might not get everything considered).

Thanks!

> 0009 is some changes made by me, I changed CopyToGetFormat to
> CopyToSendCopyBegin because pg_copy_json need to send different bytes
> in SendCopyBegin, get the format code along is not enough

Oh, I haven't cared about the case.
How about the following API instead?

static void
SendCopyBegin(CopyToState cstate)
{
	StringInfoData buf;

	pq_beginmessage(&buf, PqMsg_CopyOutResponse);
	cstate->opts.to_routine->CopyToFillCopyOutResponse(cstate, &buf);
	pq_endmessage(&buf);
	cstate->copy_dest = COPY_FRONTEND;
}

static void
CopyToJsonFillCopyOutResponse(CopyToState cstate, StringInfoData &buf)
{
	int16		format = 0;

	pq_sendbyte(&buf, format);      /* overall format */
	/*
	 * JSON mode is always one non-binary column
	 */
	pq_sendint16(&buf, 1);
	pq_sendint16(&buf, format);
}

> 00010 is the pg_copy_json extension, I think this should be a good
> case which can utilize the *extendable copy format* feature

It seems that it's convenient that we have one more callback
for initializing CopyToState::opaque. It's called only once
when Copy{To,From}Routine is chosen:

typedef struct CopyToRoutine
{
	void		(*CopyToInit) (CopyToState cstate);
...
};

void
ProcessCopyOptions(ParseState *pstate,
				   CopyFormatOptions *opts_out,
				   bool is_from,
				   void *cstate,
				   List *options)
{
...
	foreach(option, options)
	{
		DefElem    *defel = lfirst_node(DefElem, option);

		if (strcmp(defel->defname, "format") == 0)
		{
			...
			opts_out->to_routine = &CopyToRoutineXXX;
			opts_out->to_routine->CopyToInit(cstate);
			...
		}
	}
...
}


>                                                              maybe we
> should delete copy_test_format if we have this extension as an
> example?

I haven't read the COPY TO format json thread[1] carefully
(sorry), but we may add the JSON format as a built-in
format. If we do it, copy_test_format is useful to test the
extension API.

[1] https://www.postgresql.org/message-id/flat/CALvfUkBxTYy5uWPFVwpk_7ii2zgT07t3d-yR_cy4sfrrLU%3Dkcg%40mail.gmail.com


Thanks,
-- 
kou

Commits

  1. Refactor Copy{From|To}GetRoutine() to use pass-by-reference argument.

  2. Refactor COPY FROM to use format callback functions.

  3. Refactor COPY TO to use format callback functions.

  4. Another try to fix BF failure introduced in commit ddd5f4f54a.

  5. Revert "Refactor CopyReadAttributes{CSV,Text}() to use a callback in COPY FROM"

  6. Improve COPY TO performance when server and client encodings match

  7. Simplify signature of CopyAttributeOutCSV() in copyto.c

  8. Revert "Refactor CopyAttributeOut{CSV,Text}() to use a callback in COPY TO"

  9. Refactor CopyAttributeOut{CSV,Text}() to use a callback in COPY TO

  10. Refactor CopyReadAttributes{CSV,Text}() to use a callback in COPY FROM

  11. Add progress reporting of skipped tuples during COPY FROM.

  12. pgbench: Add \syncpipeline

  13. meson: Make gzip and tar optional

  14. Export the external file reader used in COPY FROM as APIs.