Re: backend_xmin in pg_stat_replication

Thanks a lot! So, the correct calculation is like this:

select application_name,
txid_snapshot_xmin(txid_current_snapshot()),
backend_xmin::TEXT::BIGINT,

(txid_snapshot_xmin(txid_current_snapshot())-backend_xmin::TEXT::BIGINT)%(2^32)::BIGINT
from pg_stat_replication;

application_name | txid_snapshot_xmin | backend_xmin | ?column?
------------------+--------------------+--------------+----------
xxxxxxxxxx | 6960964080 | 2665996642 | 142

That makes more sense.

On Mon, Oct 1, 2018 at 5:32 PM Andres Freund <andres(at)anarazel(dot)de> wrote:

> Hi,
>
> On 2018-10-01 12:20:26 +0200, Torsten Förtsch wrote:
> > if I understand it correctly, backend_xmin in pg_stat_replication is the
> > xmin that's reported back by hot_standby_feedback. Given there are no
> > long-running transactions on the replica, I presume that value should be
> > pretty close to the xmin field of any recent snapshots on the master.
> This
> > is true for all my databases but one:
> >
> > select application_name,
> > txid_snapshot_xmin(txid_current_snapshot()),
> > backend_xmin::TEXT::BIGINT,
> >
> > txid_snapshot_xmin(txid_current_snapshot())-backend_xmin::TEXT::BIGINT
> > from pg_stat_replication;
> >
> > application_name | txid_snapshot_xmin | backend_xmin | ?column?
> > ------------------+--------------------+--------------+------------
> > xxxxxxxxxx | 6 957 042 833 | 2 662 075 435 | 4 294 967
> 398
>
> I don't think the calculation you're doing here is correct.
> backend_xmin is an xid (max 2^32-1), whereas txid_snapshot_xmin returns
> an xid *with* epoch (max 2^64-1). What you're measuring here is simply
> the fact that the xid counter has wrapped around.
>
> Greetings,
>
> Andres Freund
>