backend_xmin in pg_stat_replication

Hi,

On 2018-10-01 12:20:26 +0200, Torsten F�rtsch 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

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@anarazel.de> wrote: