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

On Sun, Mar 29, 2020 at 9:01 PM Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>
> On Sun, Mar 29, 2020 at 11:19:21AM +0530, Amit Kapila wrote:
> >On Sun, Mar 29, 2020 at 6:29 AM Tomas Vondra
> ><tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> >>
> >> Ummm, how is that different from what the patch is doing now? I mean, we
> >> only write the top-level XID for the first WAL record in each subxact,
> >> right? Or what would be the difference with your approach?
> >>
> >
> >We have to do what the patch is currently doing and additionally, we
> >will set this flag after PGPROC_MAX_CACHED_SUBXIDS which would allow
> >us to call ProcArrayApplyXidAssignment during WAL replay only after
> >PGPROC_MAX_CACHED_SUBXIDS number of subxacts. It will help us in
> >clearing the KnownAssignedXids at the same time as we do now, so no
> >additional performance overhead.
> >
>
> Hmmm. So we'd still log assignment twice? Or would we keep just the
> immediate assignments (embedded into xlog records), and cache the
> subxids on the replica somehow?
>

I think we need to cache the subxids on the replica somehow but I
don't have a very good idea for it. Basically, there are two ways to
do it (a) Change the KnownAssignedXids in some way so that we can
easily find this information without losing on the current benefits of
it. I can't think of a good way to do that and even if we come up
with something, it could easily be a lot of work, (b) Cache the
subxids for a particular transaction in local memory along with
KnownAssignedXids. This is doable but now we have two data-structures
(one in shared memory and other in local memory) managing the same
information in different ways.

Do you have any other ideas?

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