Re: Physical replication slot advance is not persistent

On Thu, Jul 09, 2020 at 04:12:49PM +0530, Amit Kapila wrote:
> On Fri, Jun 19, 2020 at 12:16 AM Alexey Kondratov
> <a.kondratov@postgrespro.ru> wrote:
>> 1. Both ReplicationSlotsComputeRequiredXmin() and
>> ReplicationSlotsComputeRequiredLSN() may have already been done in the
>> LogicalConfirmReceivedLocation() if it was a logical slot.
>>
>
> I think it is not done in all cases, see the else part in
> LogicalConfirmReceivedLocation.
>
> LogicalConfirmReceivedLocation
> {
> ..
> else
> {
> SpinLockAcquire(&MyReplicationSlot->mutex);
> MyReplicationSlot->data.confirmed_flush = lsn;
> SpinLockRelease(&MyReplicationSlot->mutex);
> }
> ..
> }

Thanks Amit, and sorry for the late catchup.  The choice of computing
the minimum LSN and xmin across all slots at the end of
pg_replication_slot_advance() is deliberate.  That's more consistent
with the slot creation, copy and drop for one, and that was also the
intention of the original code (actually a no-op as introduced by
9c7d06d).  This also brings an interesting property to the advancing
routines to be able to enforce a recomputation without having to wait
for a checkpoint or a WAL sender to do so.  So, while there may be
cases where we don't need this recomputation to happen, and there may
be cases where it may be a waste, the code simplicity and consistency
are IMO reasons enough to keep this code as it is now.
--
Michael