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

On Mon, Feb 7, 2022 at 11:43 AM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > That's because, if VACUUM is only ever getting triggered by XID
> > age advancement and not by bloat, there's no opportunity for your
> > patch set to advance relfrozenxid any sooner than we're doing now.
>
> We must distinguish between:
>
> 1. "VACUUM is fundamentally never going to need to run unless it is
> forced to, just to advance relfrozenxid" -- this applies to tables
> like the stock and customers tables from the benchmark.
>
> and:
>
> 2. "VACUUM must sometimes run to mark newly appended heap pages
> all-visible, and maybe to also remove dead tuples, but not that often
> -- and yet we current only get expensive and inconveniently timed
> anti-wraparound VACUUMs, no matter what" -- this applies to all the
> other big tables in the benchmark, in particular to the orders and
> order lines tables, but also to simpler cases like pgbench_history.

It's not really very understandable for me when you refer to the way
table X behaves in Y benchmark, because I haven't studied that in
enough detail to know. If you say things like insert-only table, or a
continuous-random-updates table, or whatever the case is, it's a lot
easier to wrap my head around it.

> Does that make sense? It's pretty subtle, admittedly, and you no doubt
> have (very reasonable) concerns about the extremes, even if you accept
> all that. I just want to get the general idea across here, as a
> starting point for further discussion.

Not really. I think you *might* be saying tables which currently get
only wraparound vacuums will end up getting other kinds of vacuums
with your patch because things will improve enough for other tables in
the system that they will be able to get more attention than they do
currently. But I'm not sure I am understanding you correctly, and even
if I am I don't understand why that would be so, and even if it is I
think it doesn't help if essentially all the tables in the system are
suffering from the problem.

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