RE: speed up a logical replica setup

Dear Amit,

> Your analysis sounds correct to me.

Okay, so we could have a same picture...

> > IIUC, the root cause is that pg_create_logical_replication_slot() returns a LSN
> > which is not generated yet. So, I think both mine [1] and Euler's approach [2]
> > can solve the issue. My proposal was to add an extra WAL record after the final
> > slot creation, and Euler's one was to use a restart_lsn as the
> recovery_target_lsn.
> >
>
> I don't think it is correct to set restart_lsn as consistent_lsn point
> because the same is used to set replication origin progress. Later
> when we start the subscriber, the system will use that LSN as a
> start_decoding_at point which is the point after which all the commits
> will be replicated. So, we will end up incorrectly using restart_lsn
> (LSN from where we start reading the WAL) as start_decoding_at point.
> How could that be correct?

I didn't say we could use restart_lsn as consistent point of logical replication,
but I could agree the approach has issues.

> Now, even if we use restart_lsn as recovery_target_lsn and the LSN
> returned by pg_create_logical_replication_slot() as consistent LSN to
> set replication progress, that also could lead to data loss because
> the subscriber may never get data between restart_lsn value and
> consistent LSN value.

You considered the case, e.g., tuples were inserted just after the restart_lsn
but before the RUNNING_XACT record? In this case, yes, the streaming replication
finishes before replicating tuples but logical replication will skip them.
Euler's approach cannot be used as-is.

Best regards,
Hayato Kuroda
FUJITSU LIMITED