Alvaro Herrera wrote:
Heikki Linnakangas wrote:
Alvaro Herrera wrote:
Tom Lane wrote:
Alvaro Herrera <[EMAIL PROTECTED]> 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.
Hmm. So if we call that counter VacuumXmin for now, you could remove
deleted rows with xmax < VacuumXmin, as long as that xmax is not in the
set of running transactions? I guess that would work.
In general: VACUUM can remove any tuple that's not visible to any
snapshot in the system. We don't want to keep all snapshots in shared
memory, so we use some conservative approximation of that.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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