Re: Synchronizing slots from primary to standby

On Wed, Mar 6, 2024 at 7:36 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Tue, Mar 5, 2024 at 4:21 PM Zhijie Hou (Fujitsu)
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> I have one question about PhysicalWakeupLogicalWalSnd():
>
> +/*
> + * Wake up the logical walsender processes with logical failover slots if the
> + * currently acquired physical slot is specified in standby_slot_names GUC.
> + */
> +void
> +PhysicalWakeupLogicalWalSnd(void)
> +{
> + List *standby_slots;
> +
> + Assert(MyReplicationSlot && SlotIsPhysical(MyReplicationSlot));
> +
> + standby_slots = GetStandbySlotList();
> +
> + foreach_ptr(char, name, standby_slots)
> + {
> + if (strcmp(name, NameStr(MyReplicationSlot->data.name)) == 0)
> + {
> +
> ConditionVariableBroadcast(&WalSndCtl->wal_confirm_rcv_cv);
> + return;
> + }
> + }
> +}
>
> IIUC walsender calls this function every time after updating the
> slot's restart_lsn, which could be very frequently. I'm concerned that
> it could be expensive to do a linear search on the standby_slot_names
> list every time. Is it possible to cache the information in walsender
> local somehow?
>

We can cache this information for WalSender but not for the case where
users use pg_physical_replication_slot_advance(). We don't expect this
list to be long enough to matter, so we can leave this optimization
for the future especially if we encounter any such case unless you
think otherwise.

--
With Regards,
Amit Kapila.