Hi, On 2018-10-01 12:20:26 +0200, Torsten Förtsch wrote: > if I understand it correctly, backend_xmin in pg_stat_replication is the > xmin that's reported back by hot_standby_feedback. Given there are no > long-running transactions on the replica, I presume that value should be > pretty close to the xmin field of any recent snapshots on the master. This > is true for all my databases but one: > > select application_name, > txid_snapshot_xmin(txid_current_snapshot()), > backend_xmin::TEXT::BIGINT, > > txid_snapshot_xmin(txid_current_snapshot())-backend_xmin::TEXT::BIGINT > from pg_stat_replication; > > application_name | txid_snapshot_xmin | backend_xmin | ?column? > ------------------+--------------------+--------------+------------ > xxxxxxxxxx | 6 957 042 833 | 2 662 075 435 | 4 294 967 398 I don't think the calculation you're doing here is correct. backend_xmin is an xid (max 2^32-1), whereas txid_snapshot_xmin returns an xid *with* epoch (max 2^64-1). What you're measuring here is simply the fact that the xid counter has wrapped around. Greetings, Andres Freund
