Anything else than measured in bytes either requires a lookup at the file timestamp, which is not reliable with noatime or a lookup at WAL itself to decide when is the commit timestamp that matches the oldest point in time of the backup policy.
An indication that it'd be nice to have a better way to store this information as part of a base backup, or the archived WAL files.
That could be made performance wise with an archive command. With pg_receivexlog you could make use of the end-segment command to scan the completely written segment for this data before moving on to the next one. At least it gives an argument for having such a command. David Steele mentioned that he could make use of such a thing.
BTW, I'm not opposed to an end-segment command; I'm just saying I don't think having it would really help users very much.
It might not help end users directly, but it could certainly help tools-developers. At least that's what I'd think.
