Re: [HACKERS] make async slave to wait for lsn to be replayed

Hi!

On Wed, Oct 4, 2023 at 1:22 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> I see you're concentrating on the procedural version of this feature.  But when you're calling a procedure within a
normalSQL statement, the executor gets a snapshot and holds it until the procedure finishes. In the case the WAL record
conflictswith this snapshot, the query will be canceled.  Alternatively, when hot_standby_feedback = on, the query and
WALreplayer will be in a deadlock (WAL replayer will wait for the query to finish, and the query will wait for WAL
replayed). Do you see this issue?  Or do you think I'm missing something? 

I'm sorry, I actually meant hot_standby_feedback = off
(hot_standby_feedback = on actually avoids query conflicts).  I
managed to reproduce this problem.

master: create table test as (select i from generate_series(1,10000) i);
slave conn1: select pg_wal_replay_pause();
master: delete from test;
master: vacuum test;
master: select pg_current_wal_lsn();
slave conn2: select pg_wait_lsn('the value from previous query'::pg_lsn, 0);
slave conn1: select pg_wal_replay_resume();
slave conn2: ERROR:  canceling statement due to conflict with recovery
DETAIL:  User query might have needed to see row versions that must be removed.

Needless to say, this is very undesirable behavior.  This happens
because pg_wait_lsn() has to run within a snapshot as any other
function.  This is why I think this functionality should be
implemented as a separate statement.

Another issue I found is that pg_wait_lsn() hangs on the primary.   I
think an error should be reported instead.

------
Regards,
Alexander Korotkov