Re: decoupling table and index vacuum

On Thu, Apr 22, 2021 at 3:11 PM Peter Geoghegan <pg@bowt.ie> wrote:
> I think that everybody's beliefs about VACUUM tend to be correct. It
> almost doesn't matter if scenario A is the problem in 90% or cases
> versus 10% of cases for scenario B (or vice-versa). What actually
> matters is that we have good handling for both. (It's probably some
> weird combination of scenario A and scenario B in any case.)

I agree strongly with this. In fact, I seem to remember saying similar
things to you in the past. If something wins $1 in 90% of cases and
loses $5 in 10% of cases, is it a good idea? Well, it depends on how
the losses are distributed. If every user can be expected to hit both
winning and losing cases with approximately those frequencies, then
yes, it's a good idea, because everyone will come out ahead on
average. But if 90% of users will see only wins and 10% of users will
see only losses, it sucks.

That being said, I don't know what this really has to do with the
proposal on the table, except in the most general sense. If you're
just saying that decoupling stuff is good because different indexes
have different needs, I am in agreement, as I said in my OP. It sort
of sounded like you were saying that it's not important to try to
estimate the number of undeleted dead tuples in each index, which
puzzled me, because while knowing doesn't mean everything is
wonderful, not knowing it sure seems worse. But I guess maybe that's
not what you were saying, so I don't know. I feel like we're in danger
of drifting into discussions about whether we're disagreeing with each
other rather than, as I would like, focusing on how to design a system
for $SUBJECT.

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