Re: advancing snapshot's xmin

Alvaro Herrera wrote:
> 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?

Everything < xmin is considered to be not running anymore. Other 
transactions would consider the still-alive transaction as aborted, and 
start setting hint bits etc.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com