Re: Timeline following for logical slots

On 2016-04-04 14:36:29 +0800, Craig Ringer wrote:
> On 4 April 2016 at 14:30, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> > On 2016-04-04 14:24:52 +0800, Craig Ringer wrote:
> > > I don't feel like I've grasped this properly yet. I think it's referring
> > to
> > > the pg_logical/snapshots/ serialization, the use of which allows us to
> > > avoid doing extra work in SnapBuildFindSnapshot(...), but doesn't seem to
> > > be crucial for correct function. After all, decoding still restarts at
> > the
> > > restart_lsn and feeds relevant xact info into the snapshot builder,
> > > accumulates invalidation information, etc.
> >
> > restart_lsn is set to the last known point where a) all changes for
> > ongoing transactions are available b) we can re-build visiblity
> > information when we start reading from there.
> >
> > As we essentially can only start determining visibility information
> > whenever processing a xl_running_xacts record. Building visibility
> > information means that there has to be a xl_running_xacts to start
> > from. To build full visibility information we also have to wait till we
> > have seen all in-progress transactions finish. So we dump visibility
> > information every now and then, so we can re-use the information we'd
> > already assembled.
> >
>
> OK, makes sense. And still resume decoding from restart_lsn, despite having
> that visibility information stashed, because we also have to rebuild the
> information on invalidations for running xacts, which is not stored
> persistently anywhere as decoding progresses. So for now at least it's an
> optimisation to store the visibility info, since we still have go go back
> and decode for invalidations anyway. Right?

Not really no. The important point isn't invalidation or anything. It's
that we don't have the content & metadata of the transactions
themselves. Yes, we also do re-read invalidations, but that's just a
side effect.