Re: Add index scan progress to pg_stat_progress_vacuum

> One idea would be to add a flag, say report_parallel_vacuum_progress,
> to IndexVacuumInfo struct and expect index AM to check and update the
> parallel index vacuum progress, say every 1GB blocks processed. The
> flag is true only when the leader process is vacuuming an index.

Sorry for the long delay on this. I have taken the approach as suggested
by Sawada-san and Robert and attached is v12.

1. The patch introduces a new counter in the shared memory already
used by the parallel leader and workers to keep track of the number
of indexes completed. This way there is no reason to loop through
the index status everytime we want to get the status of indexes completed.

2. A new function in vacuumparallel.c will be used to update
the progress of a indexes completed by reading from the
counter created in point #1.

3. The function is called during the vacuum_delay_point as a
matter of convenience, since it's called in all major vacuum
loops. The function will only do anything if the caller
sets a boolean to report progress. Doing so will also ensure
progress is being reported in case the parallel workers completed
before the leader.

4. Rather than adding any complexity to WaitForParallelWorkersToFinish
and introducing a new callback, vacuumparallel.c will wait until
the number of vacuum workers is 0 and then process to call
WaitForParallelWorkersToFinish as it does.

5. Went back to the idea of adding a new view called pg_stat_progress_vacuum_index
which is accomplished by adding a new type called VACUUM_PARALLEL in progress.h

Thanks,

Sami Imseih
Amazon Web Servies (AWS)