Re: new autovacuum criterion for visible pages

On Fri, Aug 12, 2016 at 9:01 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Michael Paquier <michael(dot)paquier(at)gmail(dot)com> writes:
>> In short, autovacuum will need to scan by itself the VM of each
>> relation and decide based on that.
>
> That seems like a worthwhile approach to pursue. The VM is supposed to be
> small, and if you're worried it isn't, you could sample a few pages of it.
> I do not think any of the ideas proposed so far for tracking the
> visibility percentage on-the-fly are very tenable.
>

The one visibility map page can store the information of 32672 heap
pages (255MB), but it would be cost if autovacuum scan whole
visibility map for all tables.
So I think that it's better to provide
autovacuum_vacuum_pagevisible_factor as a relopts.
And the autovacuum scans or samples the visibility map of table that
autovacuum_vacuum_pagevisible_factor is set.

Regards,

--
Masahiko Sawada