Re: Starting logical replication at arbitrary point that's available in WAL

[Alexander Uvizhev <uvizhe@xxxxxxxxxx>]

On 12/27/24 13:20, Zhijie Hou (Fujitsu) wrote:

> On Friday, December 27, 2024 7:39 PM Alexander Uvizhev <uvizhe@xxxxxxxxxx> wrote:
>
> Hi,
>
> > I'm doing a logical replication using streaming replication protocol and
> > I'm trying to start a stream from a certain arbitrary point that's
> > available in WAL. However, both CREATE_REPLICATION_SLOT and
> > pg_create_logical_replication_slot() create slot with nearly last LSN/XID.
> >
> > Is it possible to create a replication slot with arbitrary LSN/XID values?
> >       
> I think it's not supported to specify arbitrary LSN/XID values for a newly
> created slot in core. However, if you have an existing slot with an older LSN/XID, you
> can copy it using pg_copy_logical_replication_slot, and then advance the copied
> slot to your desired position with pg_replication_slot_advance.
>     
    Thanks for the idea, I've already discovered
      it and it requires me to implement some script to advance that
      dedicated slot, which I was hoping to avoid.
> > Also pg_create_logical_replication_slot() for some reason gives
> > different result than CREATE_REPLICATION_SLOT: new slot's `catalog_xmin`
> > is set to the smallest `catalog_xmin` among already existing slots.
> > Looks like a bug.
> >       
> Could you provide a script to reproduce this issue ?
> That would be helpful in diagnosing the reason.
>

Unfortunately, I have no such script. But it looks like this:

    test=> select slot_name, catalog_xmin,
      restart_lsn, confirmed_flush_lsn from pg_replication_slots;
      slot_name    | catalog_xmin | restart_lsn |
      confirmed_flush_lsn
    ---------------+--------------+-------------+---------------------
     inactive_slot |        10073 | 0/1101DB88  |
      0/1101DB88
     active_slot  
    |        42607 | 0/17A87410  |
      0/17A87410                           
     physical      |              | 0/17A875A0  |
    
    test=> select 
      pg_create_logical_replication_slot('test_slot', 'decoderbufs');
      slot_name    | catalog_xmin | restart_lsn |
      confirmed_flush_lsn
    
      ---------------+--------------+-------------+---------------------
    
       inactive_slot |        10073 | 0/1101DB88  | 0/1101DB88
     active_slot   |        42607 |
      0/17A87410  | 0/17A87410                           
    
       physical      |              | 0/17A875D8  |
       test_slot     |        10073 |
      0/17A875A0  | 0/17A875D8

While using CREATE_REPLICATION_SLOT would give me the same slot with `catalog_xmin` = 42607.

Hope this helps.

> Best Regards,
> Hou zj
>     
    -- 
      AU