Re: backend_xmin in pg_stat_replication

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