Re: Minimal logical decoding on standbys

On Fri, 31 May 2019 at 11:08, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
>
> On Thu, 30 May 2019 at 20:13, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >
> > Hi,
> >
> > On 2019-05-30 19:46:26 +0530, Amit Khandekar wrote:
> > > @@ -1042,7 +1042,8 @@ ReplicationSlotReserveWal(void)
> > > if (!RecoveryInProgress() && SlotIsLogical(slot))
> > > {
> > > ....
> > > }
> > > else
> > > {
> > > - restart_lsn = GetRedoRecPtr();
> > > + restart_lsn = SlotIsLogical(slot) ?
> > > + GetXLogReplayRecPtr(&ThisTimeLineID) : GetRedoRecPtr();
> > >
> > > But then when I do pg_create_logical_replication_slot(), it hangs in
> > > DecodingContextFindStartpoint(), waiting to find new records
> > > (XLogReadRecord).
> >
> > But just till the primary has logged the necessary WAL records? If you
> > just do CHECKPOINT; or such on the primary, it should succeed quickly?
>
> Yes, it waits until there is a commit record, or (just tried) until a
> checkpoint command.

Is XLOG_RUNNING_XACTS record essential for the logical decoding to
build a consistent snapshot ?
Since the restart_lsn is now ReplayRecPtr, there is no
XLOG_RUNNING_XACTS record, and so the snapshot state is not yet
SNAPBUILD_CONSISTENT. And so
DecodingContextFindStartpoint()=>DecodingContextReady() never returns
true, and hence DecodingContextFindStartpoint() goes in an infinite
loop, until it gets XLOG_RUNNING_XACTS.

--
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company