Re: advancing snapshot's xmin

Tom Lane wrote:
> 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.

My point exactly -- can we let the Xmin go past its Xid?  You imply we
can't, but why?

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
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