Re: visibilitymap_count() at the end of vacuum

Hi,

On 2012-12-03 23:44:36 +0530, Pavan Deolasee wrote:
> I wonder if we really need to make another pass over the entire visibility
> map to count the number of all-visible pages at the end of the vacuum. The
> code that I'm looking at is in src/backend/commands/vacuumlazy.c:
>
>  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.

A full-table vacuum can take a *long* (as in days) time, so I think
recounting makes sense. And normally the cost is pretty small, so I
don't see a problem in this.

Why change it?

Andres
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