Re: Synchronizing slots from primary to standby

Hi,

On 9/25/23 10:44 AM, Drouvot, Bertrand wrote:
> Hi,
>
> On 9/23/23 3:38 AM, Amit Kapila wrote:
>> On Fri, Sep 22, 2023 at 6:01 PM Drouvot, Bertrand
>> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:

>> There is a difference here that we also need to prevent removal of
>> rows required by sync_slots. That could be achieved by physical slot
>> (and hot_standby_feedback). So, having a requirement to have physical
>> slot doesn't sound too unreasonable to me. Otherwise, we need to
>> invent some new mechanism of having some sort of placeholder slot to
>> avoid removal of required rows.
>
> Thinking about it, I wonder if removal of required rows is even possible
> given that:
>
> - we don't allow to logical decode from a sync slot
> - sync slot catalog_xmin <= its primary counter part catalog_xmin
> - its primary counter part prevents rows removal thanks to its own catalog_xmin
> - a sync slot is removed as soon as its primary counter part is removed
>
> In that case I'm not sure how rows removal on the primary could lead to remove rows
> required by a sync slot. Am I missing something? Do you have a scenario in mind?

Please forget the above questions, it's in fact pretty easy to remove rows on the primary that
would be needed by a sync slot.

I do agree that having a requirement to have physical slot does not sound unreasonable then.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com