Re: logical decoding and replication of sequences, take 2

On Sat, Mar 18, 2023 at 8:49 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 3/18/23 06:35, Amit Kapila wrote:
> > On Sat, Mar 18, 2023 at 3:13 AM Tomas Vondra
> > <tomas.vondra@enterprisedb.com> wrote:
> >>
> >> ...
> >>
> >> Clearly, for sequences we can't quite rely on snapshots/slots, we need
> >> to get the LSN to decide what changes to apply/skip from somewhere else.
> >> I wonder if we can just ignore the queued changes in tablesync, but I
> >> guess not - there can be queued increments after reading the sequence
> >> state, and we need to apply those. But maybe we could use the page LSN
> >> from the relfilenode - that should be the LSN of the last WAL record.
> >>
> >> Or maybe we could simply add pg_current_wal_insert_lsn() into the SQL we
> >> use to read the sequence state ...
> >>
> >
> > What if some Alter Sequence is performed before the copy starts and
> > after the copy is finished, the containing transaction rolled back?
> > Won't it copy something which shouldn't have been copied?
> >
>
> That shouldn't be possible - the alter creates a new relfilenode and
> it's invisible until commit. So either it gets committed (and then
> replicated), or it remains invisible to the SELECT during sync.
>

Okay, however, we need to ensure that such a change will later be
replicated and also need to ensure that the required WAL doesn't get
removed.

Say, if we use your first idea of page LSN from the relfilenode, then
how do we ensure that the corresponding WAL doesn't get removed when
later the sync worker tries to start replication from that LSN? I am
imagining here the sync_sequence_slot will be created before
copy_sequence but even then it is possible that the sequence has not
been updated for a long time and the LSN location will be in the past
(as compared to the slot's LSN) which means the corresponding WAL
could be removed. Now, here we can't directly start using the slot's
LSN to stream changes because there is no correlation of it with the
LSN (page LSN of sequence's relfilnode) where we want to start
streaming.

Now, for the second idea which is to directly use
pg_current_wal_insert_lsn(), I think we won't be able to ensure that
the changes covered by in-progress transactions like the one with
Alter Sequence I have given example would be streamed later after the
initial copy. Because the LSN returned by pg_current_wal_insert_lsn()
could be an LSN after the LSN associated with Alter Sequence but
before the corresponding xact's commit.

-- 
With Regards,
Amit Kapila.