Re: should vacuum's first heap pass be read-only?

On Thu, Mar 31, 2022 at 5:31 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > I agree.
>
> But in http://postgr.es/m/CA+Tgmoa6kVEeurtyeOi3a+rA2XuynwQmJ_s-h4kUn6-bKMMDRw@mail.gmail.com
> (and the messages just before and just after it) we seemed to be
> agreeing on a design where that's exactly what happens. It seemed like
> a good idea to me at the time, but now it seems like it's a bad idea,
> because it involves using the conveyor belt in a way that adds no
> value.

There are two types of heap scans (in my mind, at least): those that
prune, and those that VACUUM. While there has traditionally been a 1:1
correspondence between these two scans (barring cases with no LP_DEAD
items whatsoever), that's no longer in Postgres 14, which added the
"bypass index scan in the event of few LP_DEAD items left by pruning"
optimization (or Postgres 12 if you count INDEX_CLEANUP=off).

When I said "I agree" earlier today, I imagined that I was pretty much
affirming everything else that I'd said up until that point of the
email. Which is that the conveyor belt is interesting as a way of
breaking (or even just loosening) dependencies on the *order* in which
we perform work within a given "VACUUM cycle". Things can be much
looser than they are today, with indexes (which we've discussed a lot
already), and even with heap pruning (which I brought up for the first
time just today).

However, I don't see any way that it will be possible to break one
particular ordering dependency, even with the conveyor belt stuff: The
"basic invariant" described in comments above lazy_scan_heap(), which
describes rules about TID recycling -- we can only recycle TIDs when a
full "VACUUM cycle" completes, just like today.

That was a point I was making in the email from back in February:
obviously it's unsafe to do lazy_vacuum_heap_page() processing of a
page until we're already 100% sure that the LP_DEAD items are not
referenced by any indexes, even indexes that have very little bloat
(that don't really need to be vacuumed for their own sake). However,
the conveyor belt can add value by doing much more frequent processing
in lazy_scan_prune() (of different pages each time, or perhaps even
repeat processing of the same heap pages), and much more frequent
index vacuuming for those indexes that seem to need it.

So the lazy_scan_prune() work (pruning and freezing) can and probably
should be separated in time from the index vacuuming (compared to the
current design). Maybe not for all of the indexes -- typically for the
majority, maybe 8 out of 10. We can do much less index vacuuming in
those indexes that don't really need it, in order to be able to do
much more in those that do. At some point we must "complete a whole
cycle of heap vacuuming" by processing all the heap pages using
lazy_vacuum_heap_page() that need it.

Separately, the conveyor belt seems to have promise as a way of
breaking up work for multiplexing, or parallel processing.

--
Peter Geoghegan