Re: Replication slot stats misgivings

On Tue, Apr 20, 2021 at 7:54 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
>
> I've attached the patch. In addition to the test Vignesh prepared, I
> added one test for the message for creating a slot that checks if the
> statistics are initialized after re-creating the same name slot.
>

I am not sure how much useful your new test is because you are testing
it for slot name for which we have removed the slot file. It is not
related to stat messages this patch is sending. I think we can leave
that for now. One other minor comment:

- * create the statistics for the replication slot.
+ * create the statistics for the replication slot. In the cases where the
+ * message for dropping the old slot gets lost and a slot with the same
+ * name is created, since the stats will be initialized by the message
+ * for creating the slot the statistics are not accumulated into the
+ * old slot unless the messages for both creating and dropping slots with
+ * the same name got lost. Just in case it happens, the user can reset
+ * the particular stats by pg_stat_reset_replication_slot().

I think we can change it to something like: " XXX In case, the
messages for creation and drop slot of the same name get lost and
create happens before (auto)vacuum cleans up the dead slot, the stats
will be accumulated into the old slot. One can imagine having OIDs for
each slot to avoid the accumulation of stats but that doesn't seem
worth doing as in practice this won't happen frequently.". Also, I am
not sure after your recent change whether it is a good idea to mention
something in docs. What do you think?

--
With Regards,
Amit Kapila.