Re: advancing snapshot's xmin

Alvaro Herrera <alvherre@commandprompt.com> writes:
> As far as I can see, for the purposes of VACUUM we can remove any tuple
> that was deleted after the old transaction's Xid but before that
> transaction's Xmin (i.e. all of its live snapshots).  This means we get
> to ignore Xid in GetOldestXmin and in the TransactionXmin calculations
> in GetSnapshotData.  It would not surprise me, however, to find out that
> I am overlooking something and this is incorrect.

This seems entirely off-base to me.  In particular, if a transaction
has an XID then its XMIN will never be greater than that, so I don't
even see how you figure the case will arise.
        regards, tom lane