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

[Masahiko Sawada]

On Fri, May 2, 2025 at 11:20 PM Sutou Kouhei <k...@clear-code.com> wrote:
>
> Hi,
>
> In <cad21aodny2fhc7tp7jpn24auwkew-0yjfetzbefpwg8ych6...@mail.gmail.com>
>   "Re: Make COPY format extendable: Extract COPY TO format implementations" 
> on Fri, 2 May 2025 23:02:25 -0700,
>   Masahiko Sawada <sawada.m...@gmail.com> wrote:
>
> > The progress view can know only numbers. We need to extend the
> > progress view infrastructure so that we can pass other data types.
>
> Sorry. Could you tell me what APIs referred here?
> pgstat_progress_*() functions in
> src/include/utils/backend_progress.h?

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;

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.

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com