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Re: advancing snapshot's xmin

From: Heikki Linnakangas <heikki(at)enterprisedb(dot)com>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Gregory Stark <stark(at)enterprisedb(dot)com>, Neil Conway <neilc(at)samurai(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: advancing snapshot's xmin
Date: 2008-03-28 15:16:08
Message-ID: 47ED0BB8.8030809@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Alvaro Herrera wrote:
> Tom Lane wrote:
>> Alvaro Herrera <alvherre(at)commandprompt(dot)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

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