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. [1] part of a large patch at https://www.postgresql.org/message-id/202111222156.xmo2yji5ifi2%40alvherre.pgsql -- Álvaro Herrera Valdivia, Chile — https://www.EnterpriseDB.com/ "Find a bug in a program, and fix it, and the program will work today. Show the program how to find and fix a bug, and the program will work forever" (Oliver Silfridge)
