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

Hi,

In <CAD21AoD9CBjh4u6jdiE0tG-jvejw-GJN8fUPoQSVhKh36HW2NQ@mail.gmail.com>
  "Re: Make COPY format extendable: Extract COPY TO format implementations" on Fri, 2 May 2025 23:37:46 -0700,
  Masahiko Sawada <sawada.mshk@gmail.com> wrote:

> The progress information is stored in PgBackendStatus defined in
> backend_status.h:
> 
>     /*
>      * Command progress reporting.  Any command which wishes can advertise
>      * that it is running by setting st_progress_command,
>      * st_progress_command_target, and st_progress_param[].
>      * st_progress_command_target should be the OID of the relation which the
>      * command targets (we assume there's just one, as this is meant for
>      * utility commands), but the meaning of each element in the
>      * st_progress_param array is command-specific.
>      */
>     ProgressCommandType st_progress_command;
>     Oid         st_progress_command_target;
>     int64       st_progress_param[PGSTAT_NUM_PROGRESS_PARAM];
> 
> Then the progress view maps the numbers to the corresponding strings:
> 
> CREATE VIEW pg_stat_progress_copy AS
>     SELECT
>         S.pid AS pid, S.datid AS datid, D.datname AS datname,
>         S.relid AS relid,
>         CASE S.param5 WHEN 1 THEN 'COPY FROM'
>                       WHEN 2 THEN 'COPY TO'
>                       END AS command,
>         CASE S.param6 WHEN 1 THEN 'FILE'
>                       WHEN 2 THEN 'PROGRAM'
>                       WHEN 3 THEN 'PIPE'
>                       WHEN 4 THEN 'CALLBACK'
>                       END AS "type",
>         S.param1 AS bytes_processed,
>         S.param2 AS bytes_total,
>         S.param3 AS tuples_processed,
>         S.param4 AS tuples_excluded,
>         S.param7 AS tuples_skipped
>     FROM pg_stat_get_progress_info('COPY') AS S
>         LEFT JOIN pg_database D ON S.datid = D.oid;

Thanks. I didn't know about how to implement
pg_stat_progress_copy.

> So the idea is that the backend process sets the format ID somewhere
> in st_progress_param, and then the progress view calls a SQL function,
> say pg_stat_get_copy_format_name(), with the format ID that returns
> the corresponding format name.

Does it work when we use session_preload_libraries or the
LOAD command? If we have 2 sessions and both of them load
"jsonlines" COPY FORMAT extensions, what will be happened?

For example:

1. Session 1: Register "jsonlines"
2. Session 2: Register "jsonlines"
              (Should global format ID <-> format name mapping
              be updated?)
3. Session 2: Close this session.
              Unregister "jsonlines".
              (Can we unregister COPY FORMAT extension?)
              (Should global format ID <-> format name mapping
              be updated?)
4. Session 1: Close this session.
              Unregister "jsonlines".
              (Can we unregister COPY FORMAT extension?)
              (Should global format ID <-> format name mapping
              be updated?)

Thanks,
-- 
kou