Re: protocol-level wait-for-LSN

On Mon, 28 Oct 2024 at 17:58, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> > - The Query message sends an LSN to wait for.  (This doesn't handle the
> > extended query protocol yet.)
>
> I'd suggest adding a new message type for this, so that it works the
> same with simple and extended query. Or if you just want to wait without
> issuing any query.

I imagine a libpq interface like this.

lsn = PQcurrentLSN(primaryConn)
PQsendWaitLSN(secondaryConn, lsn)
PQsendQuery(secondaryConn, ...)

One thing I'm wondering is if the current lsn could be a read-only GUC
that is reported through ParameterStatus. Because a downside of making
it part of ReadyForQuery is that you only get a ReadyForQuery at the
end of a pipeline, while a pipeline can contain multiple commits if
you use explicit BEGIN/COMMIT in your pipeline. It might be nice to be
able to wait on those commits before you've received ReadyForQuery. On
the other hand, that seems like a rather exotic usecase that maybe is
not worth thinking about too much.