Re: [PoC] pg_upgrade: allow to upgrade publisher node

On Wed, Aug 2, 2023 at 5:13 PM Hayato Kuroda (Fujitsu)
<kuroda.hayato@fujitsu.com> wrote:
>
> > 4.
> > + /*
> > + * Check that all logical replication slots have reached the current WAL
> > + * position.
> > + */
> > + res = executeQueryOrDie(conn,
> > + "SELECT slot_name FROM pg_catalog.pg_replication_slots "
> > + "WHERE (SELECT count(record_type) "
> > + " FROM pg_catalog.pg_get_wal_records_content(confirmed_flush_lsn,
> > pg_catalog.pg_current_wal_insert_lsn()) "
> > + " WHERE record_type != 'CHECKPOINT_SHUTDOWN') <> 0 "
> > + "AND temporary = false AND wal_status IN ('reserved', 'extended');");
> >
> > I think this can unnecessarily lead to reading a lot of WAL data if
> > the confirmed_flush_lsn for a slot is too much behind. Can we think of
> > improving this by passing the number of records to read which in this
> > case should be 1?
>
> I checked and pg_wal_record_info() seemed to be used for the purpose. I tried to
> move the functionality to core.

IIUC the above query checks if the WAL record written at the slot's
confirmed_flush_lsn is a CHECKPOINT_SHUTDOWN, but there is no check if
this WAL record is the latest record. Therefore, I think it's quite
possible that slot's confirmed_flush_lsn points to previous
CHECKPOINT_SHUTDOWN, for example, in cases where the subscription was
disabled after the publisher shut down and then some changes are made
on the publisher. We might want to add that check too but it would not
work. Because some WAL records could be written (e.g., by autovacuums)
during pg_upgrade before checking the slot's confirmed_flush_lsn.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com