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

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.

Yours,
Laurenz Albe