Re: Unnecessary confirm work on logical replication

> In fact, the WAL sender always starts reading WAL from restart_lsn,
> which in turn is always <= confirmed_flush_lsn. While reading WAL, WAL
> sender may read XLOG_RUNNING_XACTS WAL record with lsn <=
> confirmed_flush_lsn. While processing XLOG_RUNNING_XACTS record it may
> update its restart_lsn and catalog_xmin with current_lsn = lsn fo
> XLOG_RUNNING_XACTS record. In this situation current_lsn <=
> confirmed_flush_lsn.

This can only happen when the WAL sender is restarted.  However in
this case, the restart_lsn and catalog_xmin should have already been
persisted by the previous run of the WAL sender.

I still doubt these calls are necessary.  I think there is a
complicated chicken and egg problem here.  Here is my logic:

1) LogicalConfirmReceivedLocation() is called explicitly when
confirmed_flush is sent by the replication client.

2) LogicalConfirmReceivedLocation() is the only place that updates
confirmed_flush.

3) The replication client can only send a confirmed_flush for a
current_lsn it has already received.

4) These two functions have already run for any current_lsn the
replication client has received.

5) These two functions call LogicalConfirmReceivedLocation() only if
current_lsn <= confirmed_flush.

Thank you for your patience.