Re: Resetting spilled txn statistics in pg_stat_replication

On Fri, Jun 12, 2020 at 11:20 AM Masahiko Sawada
<masahiko.sawada@2ndquadrant.com> wrote:
>
> On Fri, 12 Jun 2020 at 12:21, Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > > Since the logical decoding intermediate files are written at per slots
> > > directory, I thought that corresponding these statistics to
> > > replication slots is also understandable for users.
> > >
> >
> > What I wanted to know is how will it help users to tune
> > logical_decoding_work_mem?  Different backends can process from the
> > same slot, so it is not clear how user will be able to make any
> > meaning out of those stats.
>
> I thought that the user needs to constantly monitor them during one
> process is executing logical decoding and to see the increments. I
> might not fully understand but I guess the same is true for displaying
> them w.r.t. process. Since a process can do logical decoding several
> times using the same slot with a different setting, the user will need
> to monitor them several times.
>

Yeah, I think we might not be able to get exact measure but if we
divide total_size spilled by exec_count, we will get some rough idea
of what should be the logical_decoding_work_mem for that particular
session.  For ex. consider the logical_decoding_work_mem is 100bytes
for a particular backend and the size spilled by that backend is 100
then I think you can roughly keep it to 200bytes if you want to avoid
spilling.  Similarly one can compute its average value over multiple
executions.  Does this make sense to you?

> > OTOH, it is easier to see how to make
> > meaning of these stats if we display them w.r.t process.  Basically,
> > we have spill_count and spill_size which can be used to tune
> > logical_decoding_work_mem and also the activity of spilling happens at
> > process level, so it sounds like one-to-one mapping.
>
> Displaying them w.r.t process also seems a good idea but I'm still
> unclear what to display and how long these values are valid.
>

I feel till the lifetime of a process if we want to display the values
at process level but I am open to hear others (including yours) views
on this.

> will have the following columns for example?
>
> * pid
> * slot_name
> * spill_txns
> * spill_count
> * spill_bytes
> * exec_count
>

Yeah, these appear to be what I have in mind.  Note that we can have
multiple entries of the same pid here because of slotname, there is
some value to display slotname but I am not completely sure if that is
a good idea but I am fine if you have a reason to include slotname?