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

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.

We'll even still VACUUM the page if it happens to have some existing
LP_DEAD items left behind by opportunistic pruning. We don't need a
cleanup in either lazy_scan_noprune (a share lock is all we need), nor
do we even need one in lazy_vacuum_heap_page (a regular exclusive lock
is all we need).

--
Peter Geoghegan