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

On Sat, Dec 9, 2023 at 10:43 AM Hayato Kuroda (Fujitsu)
<kuroda.hayato@fujitsu.com> wrote:
>
> Dear Junagn, Sutou-san,
>
> Basically I agree your point - improving a extendibility is good.
> (I remember that this theme was talked at Japan PostgreSQL conference)
> Below are my comments for your patch.
>
> 01. General
>
> Just to confirm - is it OK to partially implement APIs? E.g., only COPY TO is
> available. Currently it seems not to consider a case which is not implemented.
>
For partially implements, we can leave the hook as NULL, and check the NULL
at *ProcessCopyOptions* and report error if not supported.

> 02. General
>
> It might be trivial, but could you please clarify how users can extend? Is it OK
> to do below steps?
>
> 1. Create a handler function, via CREATE FUNCTION,
> 2. Register a handler, via new SQL (CREATE COPY HANDLER),
> 3. Specify the added handler as COPY ... FORMAT clause.
>
My original thought was option 2, but as Michael point, option 1 is
the right way
to go.

> 03. General
>
> Could you please add document-related tasks to your TODO? I imagined like
> fdwhandler.sgml.
>
> 04. General - copyright
>
> For newly added files, the below copyright seems sufficient. See applyparallelworker.c.
>
> ```
>  * Copyright (c) 2023, PostgreSQL Global Development Group
> ```
>
> 05. src/include/catalog/* files
>
> IIUC, 8000 or higher OIDs should be used while developing a patch. src/include/catalog/unused_oids
> would suggest a candidate which you can use.

Yeah, I will run renumber_oids.pl at last.

>
> 06. copy.c
>
> I felt that we can create files per copying methods, like copy_{text|csv|binary}.c,
> like indexes.
> How do other think?

Not sure about this, it seems others have put a lot of effort into
splitting TO and From.
Also like to hear from others.

>
> 07. fmt_to_name()
>
> I'm not sure the function is really needed. Can we follow like get_foreign_data_wrapper_oid()
> and remove the funciton?

I have referenced some code from greenplum, will remove this.

>
> 08. GetCopyRoutineByName()
>
> Should we use syscache for searching a catalog?
>
> 09. CopyToFormatTextSendEndOfRow(), CopyToFormatBinaryStart()
>
> Comments still refer CopyHandlerOps, whereas it was renamed.
>
> 10. copy.h
>
> Per foreign.h and fdwapi.h, should we add a new header file and move some APIs?
>
> 11. copy.h
>
> ```
> -/* These are private in commands/copy[from|to].c */
> -typedef struct CopyFromStateData *CopyFromState;
> -typedef struct CopyToStateData *CopyToState;
> ```
>
> Are above changes really needed?
>
> 12. CopyFormatOptions
>
> Can we remove `bool binary` in future?
>
> 13. external functions
>
> ```
> +extern void CopyToFormatTextStart(CopyToState cstate, TupleDesc tupDesc);
> +extern void CopyToFormatTextOneRow(CopyToState cstate, TupleTableSlot *slot);
> +extern void CopyToFormatTextEnd(CopyToState cstate);
> +extern void CopyFromFormatTextStart(CopyFromState cstate, TupleDesc tupDesc);
> +extern bool CopyFromFormatTextNext(CopyFromState cstate, ExprContext *econtext,
> +
> Datum *values, bool *nulls);
> +extern void CopyFromFormatTextErrorCallback(CopyFromState cstate);
> +
> +extern void CopyToFormatBinaryStart(CopyToState cstate, TupleDesc tupDesc);
> +extern void CopyToFormatBinaryOneRow(CopyToState cstate, TupleTableSlot *slot);
> +extern void CopyToFormatBinaryEnd(CopyToState cstate);
> +extern void CopyFromFormatBinaryStart(CopyFromState cstate, TupleDesc tupDesc);
> +extern bool CopyFromFormatBinaryNext(CopyFromState cstate,
> ExprContext *econtext,
> +
>   Datum *values, bool *nulls);
> +extern void CopyFromFormatBinaryErrorCallback(CopyFromState cstate);
> ```
>
> FYI - If you add files for {text|csv|binary}, these declarations can be removed.
>
> Best Regards,
> Hayato Kuroda
> FUJITSU LIMITED
>

Thanks for all the valuable suggestions.

-- 
Regards
Junwang Zhao