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

On Thu, Mar 24, 2022 at 2:40 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > > This is absolutely mandatory in the aggressive case, because otherwise
> > > relfrozenxid advancement might be seen as unsafe. My observation is:
> > > Why should we accept the same race in the non-aggressive case? Why not
> > > do essentially the same thing in every VACUUM?
> >
> > Sure, that seems like a good idea. I think I basically agree with the
> > goals of the patch.
>
> Great.

Attached is v12. My current goal is to commit all 3 patches before
feature freeze. Note that this does not include the more complicated
patch including with previous revisions of the patch series (the
page-level freezing work that appeared in versions before v11).

Changes that appear in this new revision, v12:

* Reworking of the commit messages based on feedback from Robert.

* General cleanup of the changes to heapam.c from 0001 (the changes to
heap_prepare_freeze_tuple and related functions). New and existing
code now fits together a bit better. I also added a couple of new
documenting assertions, to make the flow a bit easier to understand.

* Added new assertions that document
OldestXmin/FreezeLimit/relfrozenxid invariants, right at the point we
update pg_class within vacuumlazy.c.

These assertions would have a decent chance of failing if there were
any bugs in the code.

* Removed patch that made DISABLE_PAGE_SKIPPING not force aggressive
VACUUM, limiting the underlying mechanism to forcing scanning of all
pages in lazy_scan_heap (v11 was the first and last revision that
included this patch).

* Adds a new small patch 0003. This just moves the last piece of
resource allocation that still took place at the top of
lazy_scan_heap() back into its caller, heap_vacuum_rel().

The work in 0003 probably should have happened as part of the patch
that became commit 73f6ec3d -- same idea. It's totally mechanical
stuff. With 0002 and 0003, there is hardly any lazy_scan_heap code
before the main loop that iterates through blocks in rel_pages (and
the code that's still there is obviously related to the loop in a
direct and obvious way). This seems like a big overall improvement in
maintainability.

Didn't see a way to split up 0002, per Robert's suggestion 3 days ago.
As I said at the time, it's possible to split it up, but not in a way
that highlights the underlying issue (since the issue 0002 fixes was
always limited to non-aggressive VACUUMs). The commit message may have
to suffice.

--
Peter Geoghegan