Re: Proposal: Expose oldest xmin as SQL function for monitoring

James Coleman <jtc331@gmail.com> writes:
> To my knowledge the current oldest xmin (GetOldestXmin() if I'm not
> mistaken) isn't exposed directly in any view or function by Postgres.

You could do something like

select max(age(backend_xmin)) from pg_stat_activity;

though I'm not sure whether that accounts for absolutely every process.

> Am I missing anything in the above description? And if not, would
> there be any reason why we would want to avoid exposing that
> information? And if not, then would exposing it as a function be
> acceptable?

The fact that I had to use max(age(...)) in that sample query
hints at one reason: it's really hard to do arithmetic correctly
on raw XIDs.  Dealing with wraparound is a problem, and knowing
what's past or future is even harder.  What use-case do you
foresee exactly?

            regards, tom lane