Re: ERROR: subtransaction logged without previous top-level txn record

Hi,

Dan Katz <dkatz(at)joor(dot)com> writes:

> Arseny,
>
> I was hoping you could give me some insights about how this bug might
> appear with multiple replications slots. For example if I have two
> replication slots would you expect both slots to see the same error, even
> if they were started, consumed or the LSN was confirmed-flushed at
> different times?

Well, to encounter this you must happen to interrupt decoding session
(e.g. shutdown server) when restart_lsn (LSN since WAL will be read next
time) is at unfortunate position, as described in
https://www.postgresql.org/message-id/87ftjifoql.fsf%40ars-thinkpad

Generally each slot has its own restart_lsn, so if one decoding session
stucked on this issue, another one won't necessarily fail at the same
time. However, restart_lsn can be advanced only to certain points,
mainly xl_running_xacts records, which is logged every 15 seconds. So if
all consumers acknowledge changes fast enough, it is quite likely that
during shutdown restart_lsn will be the same for all slots -- which
means either all of them will stuck on further decoding or all of them
won't. If not, different slots might have different restart_lsn and
probably won't fail at the same time; but encountering this issue even
once suggests that your workload makes possibility of such problematic
restart_lsn perceptible (i.e. many subtransactions). And each
restart_lsn probably has approximately the same chance to be 'bad'
(provided the workload is even).

We need a committer familiar with this code to look here...

--
Arseny Sher
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company