Re: Commit Sequence Numbers and Visibility

On Fri, 2026-06-12 at 12:03 +0200, Matthias van de Meent wrote:

> > But we will have collapsed T1 and T2 into a single visibility
> > event,
> > which is technically a loss of information, and we'd need to sort
> > through the nuances and implications.
>
> I don't think there are many nuances.

How would remote_apply work? The docs say:

"When set to remote_apply, commits will wait until replies from the
current synchronous standby(s) indicate they have received the commit
record of the transaction and applied it, so that it has become visible
to queries on the standby(s), and also written to durable storage on
the standbys."

For it to have become visible remotely, the commit-visible record must
also have been applied remotely. But the commit-visible record can't
even be written locally until we've met the transaction's durability
requirement. If remote_apply counts as a "durability requirement", then
we have a cycle.

We also might need to look more carefully at recovery_target_time,
which could be a more surprising manifestation of the
recovery_target_xid issue.

And when does logical decoding emit the transaction, at commit or
commit-visible? What about multi-visible records? Logical decoding
needs to come up with some serial order, but (after a crash) might not
have enough information to recreate the same visible order as the
primary.

Perhaps these are not real problems or perhaps they have some
resolution. Regardless, my main point is that we want to get away from
the need to reason through this complexity. CSNs are simple to reason
about, and that simplicity will pay off across all the dependent
subsystems: replication, PITR, etc.

> Adjusting (c) with durability waits when the snapshot encounters data
> modified by not-yet-fully-durable transactions would just add this
> option to the 'newly introduced waits' concern list, too.

I'll think about this some more, it might eliminate most of the waits
in practical cases.

Regards,
Jeff Davis