Re: Minimal logical decoding on standbys

Hi,

On 1/24/23 6:20 AM, Drouvot, Bertrand wrote:
> Hi,
> 
> On 1/24/23 1:46 AM, Andres Freund wrote:
>> Hi,
>>
>> On 2023-01-19 10:43:27 +0100, Drouvot, Bertrand wrote:
>>> Sure, so with:
>>>
>>> 1) hot_standby_feedback set to off on the standby
>>> 2) create 2 logical replication slots on the standby and activate one
>>> 3) Invalidate the logical slots on the standby with VACUUM FULL on the primary
>>> 4) change hot_standby_feedback to on on the standby
>>>
>>> If:
>>>
>>> 5) pg_reload_conf() on the standby, then on the primary we get a catalog_xmin
>>> for the physical slot that the standby is attached to:
>>>
>>> postgres=# select slot_type,xmin,catalog_xmin  from pg_replication_slots ;
>>>   slot_type | xmin | catalog_xmin
>>> -----------+------+--------------
>>>   physical  |  822 |          748
>>> (1 row)
>>
>> How long did you wait for this to change? 
> 
> Almost instantaneous after pg_reload_conf() on the standby.
> 
>> I don't think there's anything right
>> now that'd force a new hot-standby-feedback message to be sent to the primary,
>> after slots got invalidated.
>>
>> I suspect that if you terminated the walsender connection on the primary,
>> you'd not see it anymore either?
>>
> 
> Still there after the standby is shutdown but disappears when the standby is re-started.
> 
>> If that isn't it, something is broken in InvalidateObsolete...
>>

Yeah, you are right: ReplicationSlotsComputeRequiredXmin() is missing for the
logical slot invalidation case (and ReplicationSlotsComputeRequiredXmin() also
needs to take care of it).

I'll provide a fix in the next revision along with the TAP tests comments addressed.

Regards,

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