Tuple visibility within a single XID

My understanding is that all subtransactions get their own unique XID 
(assuming they need one), and that CommandId can't move backwards within 
a transaction. If that's correct, then shouldn't we be able to prune 
tuples where XMIN and XMAX match our *exact* XID (not all the extra 
stuff that TransactionIdIsCurrentTransactionId() does) and CommandId < 
CurrentCommandId?

I thought of this because of a post to -general. It's certainly not a 
common case, but it seems like there's not much downside...
-- 
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com