Alvaro Herrera <alvherre@2ndquadrant.com> writes: > On 2020-Apr-01, Tom Lane wrote: >> 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?
> Maybe it would make sense to start exposing fullXids in these views and > functions, for this reason. There's no good reason to continue to > expose bare Xids to userspace, we should use them only for storage.
+1, that would help a lot.
> But I think James' point is precisely that it's not easy to know where > to look for things that keep Xmin from advancing. Currently it's > backends, replication slots, prepared transactions, and replicas with > hot_standby_feedback. If you forget to monitor just one of these, your > vacuums might be useless and you won't notice until disaster strikes.
Agreed, but just knowing what the oldest xmin is doesn't help you find *where* it is. Maybe what we need is a view showing all of these potential sources of an old xmin.
