The Transaction metrics group provides details on:
Oldest transaction ID
Percent towards the transaction ID wrapping around
Percent towards an emergency vacuum
Oldest Transaction ID
Provides a historical view of the transaction ID (XID).
Percent Towards Wraparound
Provides historical data on whether a transaction wraparound is imminent. PostgreSQL’s concurrency control relies on being able to compare the ordering of transactions against each other, using the transaction ID. However, as the transaction ID has a limited size periodic vacuums are required to mark sufficiently older transactions as frozen (i.e. will always be compared as older than current and future transactions).
If this metric is consistently growing without periodically shrinking, it may indicate that the database is blocked from performing vacuum operations on one or more tables.
Provides historical data on how close an emergency vacuum is to being triggered. Related to preventing transaction wraparounds, Postgresql will attempt to trigger an emergency vacuum operation if it is approaching a wraparound scenario (by default, this is when 200 million transactions are still “active” / unfrozen).