On Mon, Jan 17, 2022 at 4:28 PM Peter Geoghegan <pg@bowt.ie> wrote:
> Updating relfrozenxid should now be thought of as a continuous thing,
> not a discrete thing.
I think that's pretty nearly 100% wrong. The most simplistic way of
expressing that is to say - clearly it can only happen when VACUUM
runs, which is not all the time. That's a bit facile, though; let me
try to say something a little smarter. There are real production
systems that exist today where essentially all vacuums are
anti-wraparound vacuums. And there are also real production systems
that exist today where virtually none of the vacuums are
anti-wraparound vacuums. So if we ship your proposed patches, the
frequency with which relfrozenxid gets updated is going to increase by
a large multiple, perhaps 100x, for the second group of people, who
will then perceive the movement of relfrozenxid to be much closer to
continuous than it is today even though, technically, it's still a
step function. But the people in the first category are not going to
see any difference at all.
And therefore the reasoning that says - anti-wraparound vacuums just
aren't going to happen any more - or - relfrozenxid will advance
continuously seems like dangerous wishful thinking to me. It's only
true if (# of vacuums) / (# of wraparound vacuums) >> 1. And that need
not be true in any particular environment, which to me means that all
conclusions based on the idea that it has to be true are pretty
dubious. There's no doubt in my mind that advancing relfrozenxid
opportunistically is a good idea. However, I'm not sure how reasonable
it is to change any other behavior on the basis of the fact that we're
doing it, because we don't know how often it really happens.
If someone says "every time I travel to Europe on business, I will use
the opportunity to bring you back a nice present," you can't evaluate
how much impact that will have on your life without knowing how often
they travel to Europe on business. And that varies radically from
"never" to "a lot" based on the person.
--
Robert Haas
EDB: http://www.enterprisedb.com