Re: Replication slot stats misgivings

On Wed, Apr 28, 2021 at 9:37 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Wed, Apr 28, 2021 at 12:29 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Tue, Apr 27, 2021 at 11:02 AM vignesh C <vignesh21(at)gmail(dot)com> wrote:
> > >
> > > On Tue, Apr 27, 2021 at 9:48 AM vignesh C <vignesh21(at)gmail(dot)com> wrote:
> > > >
> > >
> > > Attached patch has the changes to update statistics during
> > > spill/stream which prevents the statistics from being lost during
> > > interrupt.
> > >
> >
> > void
> > -UpdateDecodingStats(LogicalDecodingContext *ctx)
> > +UpdateDecodingStats(ReorderBuffer *rb)
> >
> > I don't think you need to change this interface because
> > reorderbuffer->private_data points to LogicalDecodingContext. See
> > StartupDecodingContext.
>
> +1
>
> With this approach, we could still miss the totalTxns and totalBytes
> updates if the decoding a large but less than
> logical_decoding_work_mem is interrupted, right?
>

Right, but is there some simple way to avoid that? I see two
possibilities (a) store stats in ReplicationSlot and then send them at
ReplicationSlotRelease but that will lead to an increase in shared
memory usage and as per the discussion above, we don't want that, (b)
send intermediate stats after decoding say N changes but for that, we
need to additionally compute the size of each change which might add
some overhead.

I am not sure if any of these alternatives are a good idea. What do
you think? Do you have any other ideas for this?

--
With Regards,
Amit Kapila.