Re: How can I change replication slot's restart_lsn from SQL?

On Wed, May 17, 2023 at 08:16:41AM +0200, Laurenz Albe wrote:
> On Tue, 2023-05-16 at 19:35 +0200, hubert depesz lubaczewski wrote:
> > > I'm working on a workaround for a bug in Pg
> > > (https://www.postgresql.org/message-id/flat/Yz2hivgyjS1RfMKs%40depesz.com)
> > > I want to create replication slot, and advance is manually, keeping it
> > > always a bit lagging behind real replication slot.
> > >
> > > I can create slot, no problem:
> > > select pg_create_logical_replication_slot('depesz', 'test_decoding');
> > > and then I can, theoretically, advance it to whatever position with
> > > select * from pg_replication_slot_advance('depesz', '...');
> > >
> > > *BUT* - it changes only confirmed_flush_lsn, leaving restart_lsn as it
> > > was.
> > >
> > > How can I advance restart_lsn of a slot?
> > >
> > > Generally my idea is to get lsn from real replication slot, subtract,
> > > let's say 1GB from it, and advance my "fake slot" to this value, this
> > > keeping always buffer of 1GB in case the bug with removed wal happened
> > > again.
> > >
> > > I will be doing this on Pg12, which can limit my options, but perhaps
> > > there is some way to do it via plain(ish) SQL ?
> >
> > if doing it via SQL is not an option, is there any way to have
> > replication slot and modify it's restart_lsn, regardless of whether it
> > will b e physical, or logical, and what tool shoudl I use.
> >
> > I tried with pg_recvlogical, and read
> > https://www.postgresql.org/docs/current/protocol-replication.html but
> > I don't see how I can make restart_lsn advance to anything.
>
> You could shutdown the server and edit the file in "pg_replslot" with
> a hex editor. Not very convenient, and you'd have to study the source
> to understand the format of the file.

OK. Shutting down production server every 5 minutes to advance the slot
isn't really an option.

How do "normal" wal consumers update restart_lsn? Is there a reason why
it can't be done using pg_rec(eive|v)* ?

Best regards,

depesz