Hi, Right now we prevent dead rows in non-shared catalog tables from being removed whenever there is a logical slot with an older catalog xmin. Obviously that's required when the slot is in the current database, but it's not when the slot is in a different database.
I don't think we can afford to iterate through the slots to determine the "current database horizon" in GetSnapshotData(), but it should not be a problem to do so in GetOldestXmin(). The latter should normally not be called at a very high frequency. I think in busy clusters this could help even when only one database is in active use, because it allows relfrozenxid in template0/1 to be advanced more aggressively. I'm writing this down as an idea, I don't plan to work on this anytime soon. Greetings, Andres Freund
