Re: Add last commit LSN to pg_last_committed_xact()

On Mon, Jan 17, 2022 at 4:34 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> On 2022-Jan-14, James Coleman wrote:
> > The logical slot can't flush past the
> > last commit, so even if there's 100s of megabytes of unflushed WAL on
> > the slot there may be zero lag (in terms of what's possible to
> > process).
> >
> > I've attached a simple patch (sans tests and documentation) to get
> > feedback early. After poking around this afternoon it seemed to me
> > that the simplest approach was to hook into the commit timestamps
> > infrastructure and store the commit's XLogRecPtr in the cache of the
> > most recent value (but of course don't write it out to disk).
>
> Maybe it would work to have a single LSN in shared memory, as an atomic
> variable, which uses monotonic advance[1] to be updated. Whether this is
> updated or not would depend on a new GUC, maybe track_latest_commit_lsn.
> Causing performance pain during transaction commit is not great, but at
> least this way it shouldn't be *too* a large hit.

I don't know if it would or not, but it's such a hot path that I find
the idea a bit worrisome. Atomics aren't free - especially inside of a
loop.

--
Robert Haas
EDB: http://www.enterprisedb.com