Re: logical decoding and replication of sequences, take 2

On Wed, Jul 26, 2023 at 8:48 PM Tomas Vondra
<tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
> Anyway, I was thinking about this a bit more, and it seems it's not as
> difficult to use the page LSN to ensure sequences don't go backwards.
> The 0005 change does that, by:
>
> 1) adding pg_sequence_state, that returns both the sequence state and
> the page LSN
>
> 2) copy_sequence returns the page LSN
>
> 3) tablesync then sets this LSN as origin_startpos (which for tables is
> just the LSN of the replication slot)
>
> AFAICS this makes it work - we start decoding at the page LSN, so that
> we skip the increments that could lead to the sequence going backwards.
>

I like this design very much. It makes things simpler than complex.
Thanks for doing this.

I am wondering whether we could reuse pg_sequence_last_value() instead
of adding a new function. But the name of the function doesn't leave
much space for expanding its functionality. So we are good with a new
one. Probably some code deduplication.

--
Best Wishes,
Ashutosh Bapat