Re: Unnecessary confirm work on logical replication

On Fri, Apr 7, 2023 at 11:06 PM Emre Hasegeli <emre(at)hasegeli(dot)com> wrote:
>
> I was reading the logical replication code and found a little
> unnecessary work we are doing.
>
> The confirmed_flushed_lsn cannot reasonably be ahead of the
> current_lsn, so there is no point of calling
> LogicalConfirmReceivedLocation() every time we update the candidate
> xmin or restart_lsn.

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.

Does that make sense?

--
Best Wishes,
Ashutosh Bapat