Обсуждение: Hot standbys now support exporting snapshots while creating a logical replication slot

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/17/logicaldecoding-explanation.html
Description:

Hi,
I'm referring to this bit at the very end of the page: Creation of a
snapshot is not always possible. In particular, it will fail when connected
to a hot standby. Applications that do not require snapshot export may
suppress it with the NOEXPORT_SNAPSHOT option.
It's not clear what the intention of this is (is it referring only to
physical replication slots?) but as it stands you can export snapshots while
creating a logical replication slot. This should have been possible starting
PG16, I've tested this on PG17.
postgres=# CREATE_REPLICATION_SLOT export LOGICAL pgoutput (SNAPSHOT
'export');
-[ RECORD 1 ]----+--------------------
slot_name        | export
consistent_point | 0/36C9F28
snapshot_name    | 00000072-0000000A-1
output_plugin    | pgoutput
postgres=# SELECT pg_is_in_recovery();
-[ RECORD 1 ]-----+--
pg_is_in_recovery | t
postgres=# SHOW hot_standby;
-[ RECORD 1 ]---
hot_standby | on
As an aside, I think NOEXPORT_SNAPSHOT is from the older syntax of
CREATE_REPLICATION_SLOT and should be swapped with SNAPSHOT 'nothing'.
Thanks,
Kevin

On 2025/05/22 20:53, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
> 
> Page: https://www.postgresql.org/docs/17/logicaldecoding-explanation.html
> Description:
> 
> Hi,
> I'm referring to this bit at the very end of the page: Creation of a
> snapshot is not always possible. In particular, it will fail when connected
> to a hot standby. Applications that do not require snapshot export may
> suppress it with the NOEXPORT_SNAPSHOT option.
> It's not clear what the intention of this is (is it referring only to
> physical replication slots?) but as it stands you can export snapshots while
> creating a logical replication slot. This should have been possible starting
> PG16, I've tested this on PG17.

The description "Creation of a snapshot is not always possible. In particular,
it will fail when connected to a hot standby." no longer seems accurate.
Since v10 (commit 6c2003f8a1b), exporting snapshots on a standby has been supported.

This description was accurate when the first patch [1] adding that description
was written, but I think we just overlooked updating it later.


> As an aside, I think NOEXPORT_SNAPSHOT is from the older syntax of
> CREATE_REPLICATION_SLOT and should be swapped with SNAPSHOT 'nothing'.

Yes, so how about the following change?

      <para>
-     Creation of a snapshot is not always possible.  In particular, it will
-     fail when connected to a hot standby.  Applications that do not require
-     snapshot export may suppress it with the <literal>NOEXPORT_SNAPSHOT</literal>
+     Applications that do not require
+     snapshot export may suppress it with the <literal>SNAPSHOT 'nothing'</literal>
       option.
      </para>

Regards,

[1] https://www.postgresql.org/message-id/CAMsr+YFjxv0T8Yi1Q=3tVdgViU2bm+fb_-XUbTfxHsnLDNSkzg@mail.gmail.com

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION