Currently there's no good way that I'm aware of for monitoring software to check what the xmin horizon is being blocked at. You can check pg_stat_replication and pg_replication_slots and txid_snapshot_xmin(txid_current_snapshot()) and so on, but that list can grow, and it means monitoring setups need to update any time any new feature might hold another snapshot and expose it in a different way.
To my knowledge the current oldest xmin (GetOldestXmin() if I'm not mistaken) isn't exposed directly in any view or function by Postgres. 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? Thanks, James
