Re: advancing snapshot's xmin

Heikki Linnakangas wrote:
> 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.

Okay. So let's say we invent another TransactionId counter -- we keep
Xmin for the current purposes, and the other counter keeps track of
snapshots ignoring Xid. This new counter could be used by VACUUM to
trim dead tuples.

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