On 7 August 2017 at 14:04, Thomas Munro <thomas.mu...@enterprisedb.com>

> On Fri, Jul 21, 2017 at 7:17 PM, Kyotaro HORIGUCHI
> <horiguchi.kyot...@lab.ntt.co.jp> wrote:
> > In vac_truncate_clog, TruncateCLOG is called before
> > SetTransactionIdLimit, which advances
> > ShmemVariableCache->oldestXid. Given that the assertion in
> > TruncateCLOG is valid, they should be called in reverse order. I
> > suppose that CLOG files can be safely truncated after advancing
> > XID limits.
> If we keep the assertion by changing the order of changes to match the
> comment like this, then don't we still have a problem if another
> backend moves it backwards because of the data race I mentioned?  That
> too could be fixed (perhaps by teaching SetTransactionIdLimit not to
> overwrite higher values), but it sounds like the assertion might be a
> mistake.
> <http://www.enterprisedb.com>

I think so - specifically, that it's a leftover from a revision where the
xid limit was advanced before clog truncation.

I'll be finding time in the next couple of days to look more closely and
ensure that's all it is.

 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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