Re: Add sub-transaction overflow status in pg_stat_activity

On Mon, Nov 14, 2022 at 12:47 PM Andres Freund <andres@anarazel.de> wrote:
> I'd go the other way. It's pretty unimportant whether it overflowed, it's
> important how many subtxns there are. The cases where overflowing causes real
> problems are when there's many thousand subtxns - which one can't judge just
> from suboverflowed alone. Nor can monitoring a boolean tell you whether you're
> creeping closer to the danger zone.

This is the opposite of what I believe to be true. I thought the
problem is that once a single backend overflows the subxid array, all
snapshots have to be created suboverflowed, and this makes visibility
checking more expensive. It's my impression that for some users this
creates and extremely steep performance cliff: the difference between
no backends overflowing and 1 backend overflowing is large, but
whether you are close to the limit makes no difference as long as you
don't reach it, and once you've passed it it makes little difference
how far past it you go.