Re: Resetting spilled txn statistics in pg_stat_replication

On Thu, Jun 11, 2020 at 12:35 AM Magnus Hagander <magnus(at)hagander(dot)net> wrote:
>
> On Wed, Jun 10, 2020 at 9:01 AM Masahiko Sawada <masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
>>
>
>> If we move these values to replication slots, I think we can change
>> the code so that these statistics are managed by replication slots
>> (e.g. ReplicationSlot struct). Once ReplicationSlot has these values,
>> we can keep them beyond reconnections or multiple calls of SQL
>> interface functions. Of course, these values don’t need to be
>> persisted.
>
>
> Eh, why should they not be persisted?
>

Because these stats are corresponding to a ReoderBuffer (logical
decoding) which will be allocated fresh after restart and have no
relation with what has happened before restart.

Now, thinking about this again, I am not sure if these stats are
directly related to slots. These are stats for logical decoding which
can be performed either via WALSender or decoding plugin (via APIs).
So, why not have them displayed in a new view like pg_stat_logical (or
pg_stat_logical_decoding/pg_stat_logical_replication)? In future, we
will need to add similar stats for streaming of in-progress
transactions as well (see patch 0007-Track-statistics-for-streaming at
[1]), so having a separate view for these doesn't sound illogical.

> The comparison would be temp_files and temp_bytes in pg_stat_database, and those *are* persisted.

I am not able to see a one-on-one mapping of those stats with what is
being discussed here.

[1] - https://www.postgresql.org/message-id/CAFiTN-vXQx_161WC-a9HvNaF25nwO%3DJJRpRdTtyfGQHbM3Bd1Q%40mail.gmail.com

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com