Vacuum backend with backend_xmin?

[Torsten Förtsch <tfoertsch123@xxxxxxxxx>]

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.

Thanks,

Torsten