I considered the argument here for a bit and I think Craig is right -- FrozenXid eventually makes it to a tuple's xmin where it becomes a burden to the caller, making our interface bug-prone -- sure you can special-case it, but you don't until it first happens ... and it may not until you're deep into production.
Even the code comment is confused: "error if the given Xid doesn't normally commit". But surely FrozenXid *does* commit in the sense that it appears in committed tuples' Xmin. We already have a good mechanism for replying to the query with "this value is too old for us to have its commit TS", which is a false return value. We should use that. I think not backpatching is worse, because then users have to be aware that they need to handle the FrozenXid case specially, but only on 9.5/9.6 ... I think the reason it took this long to pop up is because it has taken this long to get to replication systems on which this issue matters. -- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers