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