On Thu, 20 Jan 2022 at 17:01, Peter Geoghegan <pg@bowt.ie> wrote:
>
> Then there's the fact that you
> really cannot think about the rate of XID consumption intuitively --
> it has at best a weak, unpredictable relationship with anything that
> users can understand, such as data stored or wall clock time.
This confuses me. "Transactions per second" is a headline database
metric that lots of users actually focus on quite heavily -- rather
too heavily imho. Ok, XID consumption is only a subset of transactions
that are not read-only but that's a detail that's pretty easy to
explain and users get pretty quickly.
There are corner cases like transactions that look read-only but are
actually read-write or transactions that consume multiple xids but
complex systems are full of corner cases and people don't seem too
surprised about these things.
What I find confuses people much more is the concept of the
oldestxmin. I think most of the autovacuum problems I've seen come
from cases where autovacuum is happily kicking off useless vacuums
because the oldestxmin hasn't actually advanced enough for them to do
any useful work.
--
greg