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

From: Alexander Uvizhev <[email-masked]>
Date: Fri, 27 Dec 2024 11:39:02 +0000

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?

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.

I'm using PostgreSQL 16.6.

-- 
AU
||

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From: "Zhijie Hou (Fujitsu)" <[email-masked]>
Date: Fri, 27 Dec 2024 12:20:05 +0000

On Friday, December 27, 2024 7:39 PM Alexander Uvizhev <[email-masked]> wrote:

Hi,

> 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.

> is set to the smallest `catalog_xmin` among already existing slots.

Could you provide a script to reproduce this issue ?
That would be helpful in diagnosing the reason.

Best Regards,
Hou zj

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From: Alexander Uvizhev <[email-masked]>
Date: Fri, 27 Dec 2024 13:18:27 +0000

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

> On Friday, December 27, 2024 7:39 PM Alexander Uvizhev<[email-masked]> wrote:
>
> Hi,
>
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.
> 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.

-- 
AU