Vacuum backend with backend_xmin?

On Mon, 2024-06-10 at 14:06 +0200, Torsten Förtsch wrote:

Now, my question is why does a vacuum backend have a backend_xmin? I am just curious.

I'd say because it sees a certain snapshot of the database, like all other
statements.

Yours,
Laurenz Albe

Hi

On Mon, Jun 10, 2024 at 5:07 PM Torsten Förtsch <tfoertsch123@gmail.com>
wrote:

Hi,

This is a VACUUM FREEZE process.

-[ RECORD 1 ]------+--------------
pid | 129471
datid | 16401
datname | feed
relid | 1889166
phase | scanning heap
heap_blks_total | 1254901
heap_blks_scanned | 1017524
heap_blks_vacuumed | 0
index_vacuum_count | 0
max_dead_tuples | 11184809
num_dead_tuples | 0
backend_xid | <NULL>
backend_xmin | 3267908740
age | 8572

The query is:

select v.*, a.backend_xid, a.backend_xmin, age(a.backend_xmin)
from pg_stat_progress_vacuum as v join pg_stat_activity as a on a.pid=v.pid

Now, my question is why does a vacuum backend have a backend_xmin? I am
just curious.

it is the oldest transaction ID whose effects may not be
visible to the transaction running in the backend.

Since transaction IDs are stored in each row to determine its visibility,
the minimum of the "backend_xmin" of all backends determines the cut-off
point beyond which all backends will agree on the visibility of tuples.

This is for example relevant for VACUUM: it cannot remove any dead tuples
that contain a transaction ID that is not older than any backend's
Regards
Kashif Zeeshan
Bitnine Global