Re: Add index scan progress to pg_stat_progress_vacuum

> The arguments of pgstat_progress_update_param() would be given by the
> worker directly as components of the 'P' message. It seems to me that
> this approach would have the simplicity to not require the setup of a
> shmem area for the extra counters, and there would be no need for a
> callback. Hence, the only thing the code paths of workers would need
> to do is to call this routine, then the leaders would increment their
> progress when they see a CFI to process the 'P' message. Also, I
> guess that we would only need an interface in backend_progress.c to
> increment counters, like pgstat_progress_incr_param(), but usable by
> workers. Like a pgstat_progress_worker_incr_param()?

So, here is what I think should be workable to give a generic
progress interface.

pgstat_progress_parallel_incr_param will be a new API that
can be called by either worker of leader from any parallel
code path that chooses to increment a progress index. 

If called by a worker, it will send a 'P' message to the front end
passing both the progress index, i.e. PROGRESS_VACUUM_INDEXES_PROCESSED
And the value to increment by, i.e. 1 for index vacuum progress.

With that, the additional shared memory counters in ParallelVacuumState
are not needed, and the poke of the worker to the leader goes directly
through a generic backend_progress API.

Let me know your thoughts.

Thanks!

--
Sami Imseih
Amazon Web Services (AWS)