visibilitymap_count() at the end of vacuum

 247     new_rel_allvisible = visibilitymap_count(onerel);

 248     if (new_rel_allvisible > new_rel_pages)

 249         new_rel_allvisible = new_rel_pages;

We would have just scanned every bit of the visibility map and can remember information about the number of all-visible pages in vacrelstats, just like many other statistical information that we track and update the end of the vacuum. Sure, there might be some more updates to the VM, especially a few bits may get cleared while we are vacuuming the table, but that can happen even while we are recounting at the end. AFAICS we can deal with that much staleness of the data.

If we agree that this is worth improving, I can write a patch to do so.

Thanks,

Pavan