Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

On Fri, Dec 20, 2019 at 2:00 PM Kyotaro Horiguchi
<horikyota(dot)ntt(at)gmail(dot)com> wrote:
>
> Hello.
>
> At Fri, 13 Dec 2019 14:46:20 +0530, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote in
> > On Wed, Dec 11, 2019 at 11:46 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > >
> > > On Mon, Dec 2, 2019 at 3:32 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > > I have rebased the patch set on the latest head.
> > >
> > > 0001 looks like a clever approach, but are you sure it doesn't hurt
> > > performance when many small XLOG records are being inserted? I think
> > > XLogRecordAssemble() can get pretty hot in some workloads.
> > >
> > > With regard to 0002, logging a separate WAL record for each
> > > invalidation seems painful; I think most operations that generate
> > > invalidations generate a bunch of them all at once. Perhaps you could
> > > just queue up invalidations as they happen, and then force anything
> > > that's been queued up to be emitted into WAL just before you emit any
> > > WAL record that might need to be decoded.
> > >
> >
> > I feel we can log the invalidations of the entire command at one go if
> > we log at CommandEndInvalidationMessages. We already have all the
> > invalidations of current command in
> > transInvalInfo->CurrentCmdInvalidMsgs. This can save us the effort of
> > maintaining a new separate list/queue for invalidations and to a good
> > extent, it will ameliorate your concern of logging each invalidation
> > separately.
>
> I have a question on this. Does that mean that the current logical
> decoder (or reorderbuffer)
>

What does currently refer to here? Is it about HEAD or about the
patch? Without the patch, we decode only at commit time and by that
time we have all invalidations (logged with commit WAL record), so we
just execute them at each catalog change (see the actions in
REORDER_BUFFER_CHANGE_INTERNAL_COMMAND_ID). The patch has to
separately WAL log each invalidation because we can decode the
intermittent changes, so we can't wait till commit. The above is just
an optimization for the patch. AFAIK, there is no correctness issue
here, but let me know if you see any.

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