On Mon, Jan 17, 2022 at 4:34 PM Alvaro Herrera <alvhe...@alvh.no-ip.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
