Re: Removing more vacuumlazy.c special cases, relfrozenxid optimizations

On Wed, Mar 23, 2022 at 4:49 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Wed, Mar 23, 2022 at 1:41 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > It seems to me that if DISABLE_PAGE_SKIPPING doesn't completely
> > disable skipping pages, we have a problem.
>
> It depends on how you define skipping. DISABLE_PAGE_SKIPPING was
> created at a time when a broader definition of skipping made a lot
> more sense.
>
> > The option isn't named CARE_ABOUT_VISIBILITY_MAP. It's named
> > DISABLE_PAGE_SKIPPING.
>
> VACUUM(DISABLE_PAGE_SKIPPING, VERBOSE) will still consistently show
> that 100% of all of the pages from rel_pages are scanned. A page that
> is "skipped" by lazy_scan_noprune isn't pruned, and won't have any of
> its tuples frozen. But every other aspect of processing the page
> happens in just the same way as it would in the cleanup
> lock/lazy_scan_prune path.

I see what you mean about it depending on how you define "skipping".
But I think that DISABLE_PAGE_SKIPPING is intended as a sort of
emergency safeguard when you really, really don't want to leave
anything out. And therefore I favor defining it to mean that we don't
skip any work at all.

--
Robert Haas
EDB: http://www.enterprisedb.com