Horton

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: sawada.mshk@gmail.com
Cc: michael@paquier.xyz, david.g.johnston@gmail.com, tgl@sss.pgh.pa.us, zhjwpku@gmail.com, pgsql-hackers@postgresql.org
Date: 2025-09-11T05:46:27Z
Lists: pgsql-hackers 
Hi,

In <CAD21AoBb3t7EcsjYT4w68p9OfMNwWTYsbSVaSRY6DRhi7sNRFg@mail.gmail.com>
  "Re: Make COPY format extendable: Extract COPY TO format implementations" on Wed, 10 Sep 2025 00:36:38 -0700,
  Masahiko Sawada <sawada.mshk@gmail.com> wrote:

> How about another idea like we move format-specific data to another
> struct that embeds CopyFrom/ToStateData at the first field and have
> CopyFrom/ToStart callback return memory with the size of that
> struct?It resolves the concerns about adding an extra indirection
> layer and extensions doesn't need to allocate memory for unnecessary
> fields (used only for built-in formats). While extensions can access
> the internal fields I think we can live with that given that there are
> some similar precedents such as table AM's scan descriptions.

The another idea looks like the following, right?

struct CopyToStateBuiltInData
{
  struct CopyToStateData parent;

  /* Members for built-in formats */
  ...;
}

typedef CopyToState *(*CopyToStart) (void);

CopyToState
BeginCopyTo(..., CopyToStart copy_to_start)
{
  ...;

  /* Allocate workspace and zero all fields */
  cstate = copy_to_start();
  ...;
}

This idea will almost work. But we can't know which
CopyToStart should be used before we parse "FORMAT" option
of COPY.

If we can iterate options twice in BeginCopy{To,From}(), we
can know it. For example:

BeginCopyTo(...)
{
  ...;

  CopyToStart copy_to_start = NULL;
  foreach(option, options)
  {
    DefElem  *defel = lfirst_node(DefElem, option);

    if (strcmp(defel->defname, "format") == 0)
    {
       char *fmt = defGetString(defel);
       if (strcmp(fmt, "text") == 0 ||
           strcmp(fmt, "csv") == 0 ||
           strcmp(fmt, "binary") == 0) {
         /* Use the builtin cstate */
       } else {
         copy_to_start = /* Detect CopyToStart for custom format */;
       }
    }
  }
  if (copy_to_start)
    cstate = copy_to_start();
  else 
    cstate = (CopyToStateData *) palloc0(sizeof(CopyToStateBuiltInData));
  ...;
}

(It may be better that we add
Copy{To,From}Routine::Copy{To,From}Allocate() instead of
CopyToStart callback.)

I think that this is acceptable because this must be a light
process. This must not have negative performance impact.


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.